Need help designing a time-travel campaign

[A Butter Idea]

I'm going to have time travel as a main aspect of my campaign. I need to work out the exact mechanics of how time travel is going to work in my setting, just so I don't have to do some serious retconning later.

I've already decided that the ability to time travel is going to be extremely limited. You cannot choose to advance or regress by an arbitrary amount of time; instead, you have a limited selection of destinations, which are measured relative to the present day.

For example, from the present day, you have the option to go forward exactly 100 years, and not a second more or less. When you plan your return trip 100 years in the future, you can go backward exactly 100 years. So for example, you could depart on 3:00 PM, January 23rd, 2026, and arrive on 3:00 PM, January 23rd, 2126. If you then spend precisely two hours and forty-five minutes in the future before returning to your present time, you would depart from on 5:45 PM, January 23rd, 2126, and arrive on 5:45 PM, January 23rd, 2026.

The options for times that you can travel to are spaced decades apart at a minimum, so unless a particular event is so important to a character that they would dedicate a large portion of their life to rewriting the past, going back for a redo is largely infeasible. To say nothing of the dangers of paradoxes, which can be hazardous to your health if you're careless.

All that's left is to determine the "vehicle" used for time-travel. Portals carved into surfaces or otherwise permanently installed into terrain or architecture are an obvious choice; however, for my party's first time-travel journey, I want to temporarily strand them 100 years in the future, requiring them to stay and explore the environment and deal with a boss encounter, before they can then return to the present day. What would be the best way to do that?

[Bluemagetim]

This is already a really cool idea.
Aiudara corrupted by aberrant power might make a good structural gate.
maybe even have a relic that is needed to make them actually work to get back and thats what the future day boss has on them.

I remember a really horrible game called drakenguard where newgame + was literally going back in time to redo things, except they didnt tell the player this. So as you did a newgame plus the protagonist just started making different choices and it always made things worse than the first playthrough. In fact every choice got more NPCs killed by the end of that playthrough than the last until you were fighting giant man eating space babies in neotokyo on your dragon. Like I said horrible game.

[A Butter Idea]

Hmmm...

So, the present-day setting is set in one of the largest metropolises in the world. One of my players is playing an elf changeling who was raised in the city and so, unlike the vast majority of elves, does not shun city life. All of this is to say, this player character is pretty much the only elf most non-elf citizens have ever seen.

What if the time-displacing aiudara only springs to life when an elf places their hand on it?

(Or, would I be running the risk of letting one of my players hog the spotlight?)

[Bluemagetim]

You could make it so its a convergence of things.
The elf player's ancestry
Another player came across a strange object either in an opening scene or as part of their background.
Either way its these chance things coming together that awakened the ancient magic in the aiudara.

[A Butter Idea]

Okay, I think I have another idea.

The time portals you find are not constructed, but are simply free-floating time-space distortions that can be found only in out-of-the-way areas. My working backstory for them is that they are spontaneously manifesting as a consequence of some wizard experiment gone horribly wrong.

Every portal the party finds only allows travel in one direction, and closes up as soon as one party's worth of creatures travels through it.

The first portal the party finds is entirely by accident, but in the process of trying to get home, they come across a tiny helpful but capricious creature (probably a magical cat with a stat block derived from the Bakeneko) that knows where the portals show up and where they take their users. That creature will guide the party back to the present day, and will facilitate future adventures by showing them to other portals they stumble across.

I'm not planning to have the cat talk, and my players so far have been reasonably good at picking up my cues.

[Bluemagetim]

having it be phenomena that they need to learn about/predict/track down should eliminate the desire to backtrack. They could even learn early these are one way phenomena.