I've had a few ideas for games that I think would be cool, but I'd never fully develop. Here they are.
A RPG where the players build modestly powerful PCs, but then roll on a list of Famous/Epic items. They could get Excalibur, the Portal Gun, etc.
An Adventure game (Think King's Quest) where every problem can be solved by a crowbar, just like in real life! This could be a mod for an existing Adventure game, or a parody game built from the ground up.
Portal 2 got me thinking about how you could make a game where any role in a machine or organization can be held by any player. If you were the "commander", you'd want the system to work well, if you were in some other function, you may intentionally sabotaging that function for some reason or other. Ideally, people would only know who the commander was, the rest of the players would be anonymously fulfilling a function to further their own ends.