Doing things a bit backwards this week and starting with the discussion. What world do you want to play in but so far haven't? It can be from a book, a movie, a cartoon, whatever you want.
For me, I think it's probably Star Trek. I'd love to see how a game works based more off the Shows than the movies. You have strange new encounters, problems have to be resolved with diplomacy and peace more than guns and action. I think it'd be heavy RP, and maybe not mechanically rewarding, but it could still be fun.
I have some friends around here who play in a Star Trek game, and it sounds like a lot of fun - though not necessarily the type of game for me for other reasons.
What about you?