Single Target, non-spell Hunter's Mark Replacement
A ground-up redesign of the concept of marking a target, based off of the Mike Mearls Happy Fun Hour, the spell itself, and Favored Foe from Tasha's. I also drew heavy inspiration from other homebrew/ 3pp versions of Hunter's Mark features, specifically Tales of the Valiant, SW5e, DracoDruid's Focused Ranger and Laserllama's Alternate Ranger. Hunter's Mark as a class feature is by no means an original idea of mine, rather this is my attempt to amalgamate various "core Marks" from both official and non-official sources into a single cohesive idea.
*(*Note: For this theoretical redesign, I don't care about compatibility with WOTC material in any way. This class feature would replace any reference to the HM spell and all features related to it, as it is meant for a hypothetical ground-up rebuild of the Ranger that is built around an actual core feature instead of a poorly designed 1st-level spell disguised as one. Subclasses would be redesigned around simply adding rider effects to the scaling core mark instead of adding extra damage.)
Marked For Death
As a bonus action, you can designate a single creature you can see within 90 feet of you as your Quarry. Alternatively, you can mark a creature by hitting it with an attack roll or by studying its tracks or other traces of its presence for at least 1 minute. Regardless of how you designate your Quarry, the target remains marked until it dies, or until you become Incapacitated, finish a short or long rest, or use this feature again to mark another creature.
Until your mark ends, whenever you damage your quarry with a weapon attack or a Ranger spell that requires an attack roll, you increase that damage by 1d4. This die, known as your Quarry Die, scales when you reach certain Ranger levels: to 1d6 at 5th level, to 1d8 at 9th level, to 1d10 at 13th level, and to 1d12 at 17th level. (Could also go with the 5.5 Monk die scaling starting at 1d6, but I am being conservative initially.)
In addition, you can add your Quarry Die to any Wisdom (Perception) or Wisdom (Survival) checks to locate your Quarry, as well as to any Intelligence checks you make to learn or recall information about it.
You can use this feature to designate quarry a number of times equal to your Wisdom modifier plus the amount shown for your level in the Ranger table. You regain one expended use when you finish a short rest and all expended uses when you finish a long rest.
Starting at 5th level, when you fail a saving throw caused by one of your Quarry's spells or innate abilities, you can use your reaction to roll your Quarry Die and add the result to the roll, potentially turning that failure into a success.
Starting at 11th level, when your Quarry is reduced to 0 hit points, you can automatically transfer your mark to the next creature you hit with an attack during your turn. Once the second target is reduced to 0 hit points, your mark automatically ends.
Starting at 17th level, once during your turn if you miss a weapon attack against your Quarry, you can make one additional weapon attack against it.