I was typing my answer to someone's question about how I managed to upscale the adventure up to Level 20 (here) but the answer was so long and passionate that I decided to create a post just for this.

I kind of went into some details while trying to not bore you. Do not hesitate if you have questions, though!

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THE MAIN THREE RULES (I am assuming that you know the module very well)

1/ The characters gain 2 levels instead of 1 (and will start at level 6).

• They can level up when they reach a new Hag domain (or the Palace), when they deal with a Hag in any way, and/or when they accomplish big achievements with the main quests for the Unicorn Horn, Snicker-Snack, or important NPCs (Agdon, Downfall, Sir Talavar, Will of the Feywild, Lamorna/Elidon, Korred/Brigganock alliance, reuniting the Twins, etc.)
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• They play a one-shot 8 years in the past as young characters going on a holiday trip to the Carnival, with no levels. Use Travis Woodall's "Lost Things: a prelude adventure to the Wild Beyond the Witchlight" homebrew content for this, it's very good.
  • Zarak is the one who stole the players' tickets while telling them in their face that they should take care of their belongings. The young heroes see each member of the League enter before them (they are here to take the portal to Prismeer thanks to the Hags).
  • Many children are spotted going without a ticket and it does not seem too bad. Make the Carnival irresistible, the young heroes have no money and cannot fathom missing their only shot to visiting the Carnival. They enter the Carnival without a ticket. Play some minigames and get their Lost Things stolen one by one. Sowpig trades candy, the lornling goes super fast, Gleam's shadow is seen under bright lights.
  • Valor's Call is seen during that time too. Helping the young heroes, advising them on the minigames, but they are pressed for time (they are tailing the League of Malevolence using hourglass pendants, gifted by the 3 Hags, as a timer during which they must find them or else they would escape. The Hags want a fight between the two parties so that they can distract Zybilna and use the cauldron). In the end, the young heroes see Valor's Call Plane Shift to somewhere, but are then caught by the security and meet briefly with Mr. Light and Mr. Witch, terrified by what they did to themselves before they are expelled.
• Then, we skip to the present time, so 8 years after the one-shot (it is fun to see the contrast on how their promising lives changed for the worse since). Follow the module but increase all DCs by 2.
• Hither starts at level 6 then there is one level up possible, and the last one occurs if Bavlorna is dealt with in any way (so potentially, they finish level 10). Increase all DCs in the module by 3.
• Tither is tailored for level 10 to 14, with the second level up possible only if Skabatha is dealt with. Increase all DCs by 4.
• Yon is level 14 to 18, same principle. Increase all DCs by 5.
• Palace is level 18-20, basically the end game. I want my players to enjoy some time at level 20 so if they do a lot before reaching the Palace, they could sneak, scout and steal at level 16, and make the final attack at level 20 against all enemies there and with a repaired Snicker-Snack, the Unicorn Horn, their Lost Things, etc.

2/ Take into account high-level Spells and features such as Wind Walk**,** Wish and Channel Divinity spam.

• The main quest is to understand what happened to Prismeer, where Zybilna is, how to free her, and as a bonus, who she really is. High-level Spells are your friends if the party is stuck. Legend Lore is very fun. Wish is their last attempt to resolve long-term afflictions or find the last bits of answers.
• Diamonds are absurdly rare, encouraging dealing with Hags. Some can be found at the Palace too.
• M components are spread out to the right places and right people (usually in a Hag's hoard or at the Palace). Some components (for narrative-breaking Spells) cannot be found at all.
• Hag Curses can be cured only by Greater Restoration if minor, or Wish if major (breaking one of the 3 rules for example), or by another Hag by making deals. Remove Curse is not enough.
• Encourage traveling by foot out of the game and with in-game incentives. A CR 26 or even CR 30 Jabberwock roaming around will do that. Air Elementals in the cloud walls, Tornadoes and storms in Yon, etc. Without a guide and if using a hex map (UFO and games' Patreon has them), they take 1d4 hours per hex (Feywild space distortion thing). This encourages finding the guides in each domain.
• Characters cannot teleport through any cloud walls unless they add their names in Tasha's ledger or if a Hag allows them to go in or out of their domain (usually because of a quest and under specific conditions). If they have the correct guide, they can walk to the next area as normal.
  • For Word of Recall, only some places are eligible and you still cannot pass the cloud walls without permission: Slanty Tower, the Unicorn Lake, the Korred Cromlech, and the Standing Stones in the Palace's garden.
• Each hag must have a way to Counterspell or Dispel Magic, move fast similar to Misty Step, and ultimately Plane Shift. Of course, you have to add Legendary Actions and Resistances and potent minions.

3/ Upscale the narrative threat of every plot, the CR of all monsters and Bosses, and give ways to loot/negotiate randomly generated lists of magic items up to the Very Rare rarity. Oh boy, here we go:

• Bavlorna's themes are the present time, gluttony/greed, and a perverted way to see the Rule of Ownership. She does not care what happened before or what happens next. She wants to relax in her pool, eat everything, take part in her hobbies and keep her sisters away. Including that pesky cat Gloam.
  • She is coached by Zargash to learn Undead Taxidermy as a hobby.
  • The snakes at Slanty Towers are changed to be a Bifid taxidermized snake of her creation. The first head creates Poisonous clouds, the second head ignites them. Kaboom.
  • Anything from a big creature could be negotiated with her, but she will use it to create other taxidermized guardians to defend her domain or herself! This includes Clapperclaw's head (creating a Leshen).
  • Lornlings are spying, Agdon is happy to serve her rather than being trapped and punished by Zybilna. He is the best around and always says so when meeting the party.
  • She is becoming paranoid of the Downfall bullywugs that according to Zargash are plotting to overthrow her. Animating their king's head is not enough. She is looking for advice.
  • Every object in her domain is hers and you should be grateful she is letting you keep some because the rest is in her collection under her roof. She would kill for Catoblepas' Milk so she can get Death Cheese (it improves the damage dealt by her poisonous breath and gastric acid).
• She receives people in her chambers as if she was a creepy psychotherapist. If you got something in your life that bothers you in the present time, she can meet you and help you, for a price. Something tangible (a heart, an eye, a hand, a loved object, etc.).
• In combat, she fights head-on with insane STR and CON.
  • She does not really want to strike you, she wants to eat you.
  • She is pretty slow on the ground, but very fast underwater. She can jump around the kung fu BBEG in Crazy Kung Fu, going through walls and floors if needed.
  • The entire room where the pool is will flood in a few rounds (hello underwater combat!) and the entire house is trying to sneeze you out by a window or door.
  • She generates lornlings like dead skin cells and can eat them to heal.
  • Her maw can extend like a snake, and her stomach is a 20 x 20 ft. dimensional space (very good to hide items or kill people) with a big glottis you can strike to get vomited out. She can eat Creatures up to Large. Creatures must resist the very strong gastric acid and fight any other creature that was swallowed too, such as the lornlings.
  • Her staff (see her official art) is a homebrew Staff of Shattered Sight, allowing her to use many sight-based Divination spells to look around her domain (but not hear) from her jacuzzi, but its 8 charges can also be used to cast a version of Counterspell that can reflect back the spell to the caster. ADV on CHA Saves but DIS on WIS Save when attuned to it to show that the paranoia comes from too much stalking without hearing what is said.
• Skabatha's theme is the past, she is the granny. Her domain is a forest made of gargantuan trees and CR 12+ beasts with some candy sprinkled here and there. She has a perverted interpretation of the Rule of Hospitality. She welcomes lost creatures and people to "educate" them on the benefits of working very hard to get a warm bed and a filling meal.
  • She is very good at cooking and gardening, using her secret fertilizer everywhere (Jabberwock dung, extremely valuable to her). The meals are very tasty and very filling (often morphing into food Mimics and Oozes), to the point that people eating them start becoming giants. This includes the children, the green dragon she found, but also the entire forest is now gargantuan trees and beasts.
  • The children use Ogre or Hill Giant stats for example. Because of the work, meals, and sleep, their body stayed young but their minds aged several years since the Hags won. The young green dragon is now very adult physically, but not mentally.
  • Her candy can Charm you to go to Loomlurch. She has a Gingerbread house in her forest (uses the Haunting Revenant stats as inspiration) with a lot of free candy.
  • Her factory is a front. In reality, she is creating a secret army of murderous toys to defend her from her sisters and from external threats. The sugar is in fact disguised gunpowder and candy-looking firearms and missile launchers are created by her genius.
  • Will of the Feywild is trying to sabotage the creation of this secret army. He wants to find blueprints, find sugar gunpowder, or even steal actual candy firearms for his gang. If they manage to get their hands on such firearms, the elf girl uses a candy sniper, the dwarf uses two pistols, and the orc uses a shotgun. While William uses a Gatling Gun. Yeah. Time to free the children!
• When dealing with visitors, she cannot stand seeing someone malnourished, weak or ill (anyone with STR or CON 8-9) and will insist they taste her food or take some candies on the way out. She acts as a kind and generous fairy godmother.
  • If you have regrets, she will be happy to make you forget them using her strange meals. She will try to use you to go on errands on her behalf since she is too busy and too "old". Catching Will of the Feywild. Or finding that Unicorn Horn. Or anything that could secretly enhance her toy army or create fertilizer. Lightning in a bottle or a Wish Stone from Yon. The glass eye Bavlorna's lornling stole from her in her sleep (she is missing one eye). Etc.
• In combat she has insane INT and WIS, and acts as that angry granny throwing waves of toys on you to kill you while she hides behind them miniaturized. She has a +2 Rolling Pin to flatten you if you come close, uses her white powder to put you to sleep, seeds to grow walls of thorns and giant beans. She has a green dragon, the rocking horse and her invulnerable dollhouse if she needs to flee.
  • Her Misty Step-like ability is her ability to miniaturize herself, gaining a lot to AC and DEX Saves and the ability to move under Medium Creatures.
  • Against magic she can use Dispel Magic (she uses a giant eraser for that) and Incapacitate foes.
• Endelyn's theme is the future. She is obsessed with avoiding her doom. While distracting herself with the doom of others in Motherhorn. Spooky lightning theater kid looking for appreciation and fishing for compliments on what she built. She has a perverted interpretation of the Rule of Reciprocity. Your fate and life are of far less concern and value compared to hers.
  • You could work years for the smallest deal and she is a very good deceiver. And if you die, she will still find use of what is left of you: a shadow or a mask.
  • She spared Brigganocks but they had to help her build the Oracle of Tragedies and the Lightning Rods. She kidnapped their architect to do so as if it was already decided.
  • She spared the Korreds in exchange that they give her hair. She did not really ask and used her scissors.
  • One day, an Ancient Blue Dragon asked her to be the most powerful lightning master to ever live. She obliged. She cut his shadow and gave that to the League to defend the Palace (= a Shadow Dragon). The rest became a CR 27 lightning spirit in the sky (= Cradle of the Storm Scion) responsible for the entire storm covering her domain, the tornadoes and the rain (cannot Long Rest if sleeping exposed to the sky, cannot fly without hover). The party could beat it using the Storm balloon, the Korred dance, and lightning resistance. Or riding the Goat, reskinned as cyclopean bronze constructs gifted with future sight.
  • Stagefright is an upscaled Nilbog with a Rod of Rulership and the ability to cast some Enchantment spells with no components.
  • The beekeeper cyclops uses Storm Giant stats for example.
• When dealing with visitors, she wants to evaluate their taste in music, in drama. The more dark their humor, the better. And if they compliment her, she is in a good mood. She pretends that she knows everything about them, which is probably partially true at this time. She will make deals that seem easy to accept, with good benefits. The twist is that she knows some obstacles will make benefiting from that deal impossible or short-lived.
  • Find my cat Gloam and bring it alive. She knows it's dead since Bavlorna taxidermized it. Oh, and the cat is a Cheshire Cat using Etherealness/Blink and Rakshasa stats. So good luck finding it and catching it.
  • Bring me 8 black roses from the Palace. She does not tell you that those roses are in fact, the head of a Rose Hydra Bush with 8 different colored heads, and that you will need to survive 8 rounds against it to harvest 8 flowers.
• In combat she is the best spellcaster of the three Hags. Good DEX, insane Mental Stats especially CHA. But a lower CON/HP pool than expected from a CR 21+ because the whole key is making an eclipse during the fight.
  • She can climb on walls like a spider.
  • She uses Foresight abilities, thread-like spells to puppet you or restrain you, infinite waves of Shadows and kamikaze Darklings. Psychic pressure and Lightning Spells for pure damage.
  • Counterspell, Dispel Magic, Blink, Mirror Image, Mind Blank, Death Ward to defend herself. She cannot teleport though, only Plane Shift. Because of that, if you can prevent her to go Ethereal or follow her there, she might be screwed.
  • She has 3 masks (Anger, Fear, Sadness) that levitate and spam Magic Missiles and one thematic Spell until dealt with.
• The big end fights will be the Heroes + Valor's Call vs the League of Malevolence and their Shadow Dragon + Jabberwock. They'll have to choose which to fight, unless they free Tasha/Bloody Beak first.
• The hidden plot is that Baba Yaga is here too, watching all of them at the Inn at the end of the Road, disguised as the druid and spectating everything.
  • She asked Scylla to steal the Unicorn Horn in order to try and teach her daughters that family and unity is far stronger than being egoistical and alone (basically, stay in a coven or be almost killed by some heroes, and you cannot escape your family duties). She does not want her daughters killed and will intervene if necessary, acting as a secret ending BBEG.
• Valor's Call and the League of Malevolence are Level 20 parties. They all have insane gear and power but you should still use NPC stat blocks (upscaled to CR 12-16) as a foundation on how they fight and what they are good at. They make Death Saves just like the players.
  • Zarak is a Thief using Magic Items and Potions he got from the Hags. He uses boots of speed, smoke bombs, Potions of Speed, Potions of Invulnerability, Potions of Invisibility, Potion of Flying, Carrion Crawler Mucus and magical daggers. He cannot one-shot a Unicorn but can hurt it badly, and should be able to escape a level 10+ party as long as he stays out of sight/run out of range. He is looking for the Unicorn Horn.
  • Zargash is a cowardly necromancer priest. Mace of Terror, Spellguard Shield. He teaches Bavlorna taxidermy so she creates Undead abominations. He will advise Bavlorna to replace the bullywug monarch with its Undead puppet version so that they stop plotting a coup. If that succeeds, he will use that puppet to create a cult to Orcus while continuing to whisper in Bavlorna's ear. Will Word of Recall faster than Lucky Luke if confronted alone and is a big healer/necrotic damage dealer in team fights.
  • Kelek is a Sorcerer using Evocation Spells and twinned Hold Person/Monster Spells. This is the disintegrate guy. Also the Forcecage/Wall of Force guy.
  • Scylla is an Archfey Warlock that spams Eldritch Blasts from very far away using Spell Sniper with Eldritch Invocations that pushes or pulls on hits (Agonizing, Repelling, Eldritch Spear). And she can teleport each turn using her staff/special Archfey Misty Steps. She uses her acting career to secretly monitor the Hags for Baba Yaga. If disturbed, she would rather go back with the League than help the party if they are obviously acting against the Hags' interests.
  • Warduke has the Flame Tongue and is a mix main Fighter/dip Zealot Barbarian. His shield is a +2 Shield of the Cavalier. His helm can terrify creatures for 1 min or until saved. He has Sentinel and Shield Master. He comes to you pushing people or making them run, and he does not leave you until you die or until his god allows you to finally kill him.
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  • Molliver lost track of time when she pursued Zarak fleeing from the palace, using her dagger of Blindsight to find him even in smokescreens. When the wall appeared she was in current Yon, and wishing to protect the Brigganocks from the floods and from Zarak, she did that for 8 years (Feywild/Brigganock time distortion things and a Wish come true, she thinks only a few days passed).
  • Elkhorn was recently freed by Scylla. When she stole the Unicorn Horn she wanted to see if it really worked. He has no idea what happened and stumbled in the frozen Palace dizzy before collapsing in front of Strongheart's frozen figure. He then woke up captured in Skabatha's kitchen with only one idea: save his friends in the palace, but the Palace is far, so first, protect those children from Skabatha. Scylla chose him because he was old and could not perceive well (and freed, he is not a threat to anybody). She brought the Unicorn Horn to Baba Yaga, disguised as the owner of the Inn at the end of the Road.
• The Lost Things become Very Rare magic items tailored to their owner and what specifically they lost. Encourage players to design them with you.
  • Sir Talavar's gifts in front of the Palace will be potent Magic Items depending on what the party asked for.
• No Legendary item can be bought, they can only be found in their relevant plot in the module (Snicker-Snack, Staff of Power, the Cauldron, etc.).
  • Snicker-Snack was broken by the Hags in three shards. Each is able to talk, but only speaking 1 word out of 3. The shards are 1) behind a 3-lock door each under Arcane Lock in the Slanty Tower with an Antimagic Field inside, 2) given by Lamorna if Elidon is brought back, and 3) lost in the Wish Stone pit by the Brigganocks. The Brigganocks can reforge the blade easily if the party promises to free Zybilna, but the party can also glue it together using Skabatha's added Sovereign Glue in Loomlurch.
• The Wells at high tide in Hither are basically Magic Item casinos (they float on the geyser). Use the DMG'25 magic item tables and roll for each hero to see what is available to trade. They can use the rule of Reciprocity 1:1 if same rarity, or 3:1 if their items are of inferior rarity.
  • If they steal, the Will-O'-Wisps attack, and they have to fight the spirit of the Well responsible for the tide. In my game, it's a Malboro boss from Final Fantasy games inspired by Shambling Mounds that heals when taking Lightning (from the Will-O'). If killed, no more high tide nor magic items and Bavlorna will investigate why her pool is not filling anymore.
• Charm and her Storm Balloon have a random inventory of items just like the Wells (see above).
• Nib can craft gold magic items of their choosing but he uses his gold pool to "pay" for them.
• Make the Wish Stones do something potent depending on what they asked Northwind during Hour 1 of the Carnival.
  • Then, spent Wish Stones can be crafted into any gem of 1000 gp+ value by the Brigganocks, but remember that the rule of Ownership and Reciprocity is key to limit access to such resources.
• Use the Domain of Delight supplement for inspiration on deals and curses. With major curses, always give a positive and a negative effect because they'll have to deal with it until level 18 or until they deal with another Hag. Here are some examples of Major Curses:
  • Bavlorna revived you but you are now a Reborn and look taxidermied until revived using Reincarnate or similar Spells.
  • Bavlorna stole your heart and is using it to create another taxidermied abomination to guard her domain that specifically loves (targets) you. You cannot feel love until you get it back.
  • Skabatha painted the whole party with gold paint. AC +2 but Speed -10 ft. and DIS on Stealth.
  • Endelyn took all your Shadows. This will create a Dark version of the party that replicate their stat blocks. and will guard her.
• If the party TPKs, they are captured by the nearest Hag. If they were trying to kill a Hag, the hags will negotiate 3 small quests they must accomplish in 8 days to redeem themselves to be on neutral grounds again.
  • If they fail to do so, they are cursed (a Major One) and will be killed on sight if they return in her domain.
  • If they failed some minor deals, curse one character. Not being able to lie, to attune to items, etc.

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I hope that this guide gave you a lot of ideas for your campaigns, even for lower levels.

I intend to end this whole thing with Zybilna telling the party that she can give them back what they truly wish for: "a scar they can still feel today, something they had but will never get back". I hope you noticed the play on present, past, future! If the party accepts, she teleports them back in time thanks to Feywild time distortion shenanigans, 8 years before, in front of the Carnival, with their tickets in hand and no League of Malevolence in sight.

It is unironically the last sentence in the last chapter of this book that made me want to run it.