Thoughts about Timeloops in videogames and TTRPGs
I have been playing a ocassional timeloop game the last year or two. (Terraformental, if you want to take a look for yourself. I can only recommend it). And I think I grasped some core ideas of the time loop game design that might be usefull for anyone trying to make a Timeloop TTRPG Adventure. Unfortunately I am not enough of a GM to actually try those ideas in practice.
You definitely need notes
Notes about plot points, enemies, where important items are.
Even a single player game needed those, a TTRPG with multiple players and sessions will need it even more. And it should be one of the better lore management tools, as this will be relevant. Don't expect player to do this as they don't even know what is actually relevant.
Different routes need gating
If you want to have multiple routes, gating some stuff at first may be necessary. There are only two things that transcend the loop in Terraformental:
The Artifacts the game is based around
Information. Mostly Pin Codes, the names and routes to entire facilities. But since the PDA loops with you it can be the functions of Key Cards, other PDA's you find and entire programms that are normally stored on physical mediums.
It is worth pointing out that some people play without the PDA and Artifacts, just for a challenge. Hard gates are usually based enitrely around memory, while those two just offer shortcuts.
Time and resource limit
In Terraformental there was always a limit (when there wasn't, it got boring). Vital Resources were the first limit. Water, Food, Air all made sense for that game. But rover power was the fallback, for travel segments.
On the resource side, Power Couplers are a limit. You need them for numerous things - repairing certain buildings, using/setting up the Artifacts for next loop, solar panel mounts. And there are never enough of them. At first it was only 1 spare in Talos. Then a second one at the dam. 3rd and 4th with Astrape (but really far apart, on different routes).
And then there is Santorini which just explodes like clockwork.
For a TTRPG, I would say that a timeloop might be the one case where you can use a Gauntlet design - no or limited rests. But only becauase of the loop and fail forward.
Be exceptionally carefull about adding or allowing anything that improves those numbers without expending other resources!
One of the strongest artifacts in the game by far is the matter artifact. On the face "+25% resources" sounds boring. But that boost is so massive, Water and Air become non-issues while in the rover and food and power goes so much furhter. It fundamentally changes the math of entire segments. It was nerfed once (down from 50%) and will probably be nerfed again.
For TTRPG's, it would be stuff like: consistent Healing like a Pearly White Ion Stone or damage mitigation like SF2 Forcefields, Resistance (both Resistacne Charms or even Ancestry or physical resistance) can massively change the math. Consider limiting how much each PC is allowed to bring here (as you do not want to invalidate whole builds), but maybe exclude core class Features (like the Guardians Physical Resistance). Single use items are lower impact, but you might want to limit them too or keep to stuff they can find on specific routes. Just don't be surprised if the backtrack in a unexpected way. Ideally say you allow those things only provisionally, assuming they don't ruin the game.
Familiarity and Fail Forward Loops
In Terraformental, the game tracks how often you did each action. The first time you finish it is a massive speed up (x1.2). But there is also a slower growing one for the same and similar actions (varies by +0.05 or 0.01 depending on how spamable it is). There is also a "previous progress" speedboosts that largely mirros the "first completion" boost, but only for the parts you did before. As you accumulate successes, eventually stuff gets put into combined actions which can do a lot of stuff with a single click - it even comes with it's own additional speed up on top of the "I did that often" ones. Because those actions just get boring.
As for TTRPG, I would implement this by progressively decreasing the difficulty and eventually skipping entire scenes, as the PC succeed them or accumulate enough tries.
Before winning any combat or skill challenge for the first time, it should have the strong adjustment for enemies. Or the hard adjustment for Skill challenge DC.
After the first completion, that drops to no Adjustment.
After third completion, this drops to Weak/Easy.
After the 4th completion, it think it would be best to just skip the scene outright. It is a solved problem, the player probably no longer enjoy it and it only costs playtime. The characters have learned the enemies failures so much, it no longer makes a relevant difference in resources.
If the PC's keep failing, eventually you should awared progress on this as well. It is said "we learn more from failures than our successes", but mostly this is about avoiding them getting stuck on something that is clearly not working out for them.
The result is that loops are "fail foward" by design. Even if you fail the final fight again this loop, next time the preceeding fights are easier or skipped (draining less resources either way). And after enough tries, the difficulty actually goes down until you can get past.
I hope I did not forget any important elements in this writeup and that people that are better GM's can actually make something of my notes.