Something that I've always wanted to do is a time-loop scenario where people play different versions of the same character, both from the past and from the future. The idea would be for players to exchange information so they can piece together what triggered the time loop and how to resolve it, but the real challenge here would be having to represent the same character in different stages of their life.

It seems like too much of a hassle though; firstly, organising everything and getting the right group of players will be no small undertaking and secondly, you have to come up with an interesting plot to tie it all together and avoid the numerous cliches from Sci-Fi shows and stories.