3D time travel and multiplayer time travel haven't really been explored.

I think there's a lot you can do within the constraints of possibility, although it may not seem that way.

I'm particularly enamoured with the interpretation of time travel where you are only free to change facts about the world that don't break prior observations.

The puzzle game Loophole, and another game which I can't mention without spoiling it, explored this idea.

This wouldn't be plausible in the real world because of the butterfly effect, but in a game world it's natural to simplify and constrain the side effects of any actions.

I'm not sure how to make the concept into a game, but I'd like to make a sandbox of this. Maybe next year...