Here is something I have never thought while playing a game, "Gee! I wish I could use my butt to control this!" Even if it's something you have never thought of, an engineer at Valve, Ben Krasnow, put together two very interesting controllers.
The "posture-based" game controller is sat on, and the player can lean in four directions to control a game. The immediate thing I thought of was using this to control leaning in a first-person game. I know I am guilty of leaning in real life when my character in a game needed to see around a corner. This would just make the leaning actually do something.
The other controller is a mouse that is controlled by your tongue, which is interesting, but I don't see that it has a lot of practical applications. I can see it being used by disabled gamers, which would be awesome if this allowed more people to play games that would physically be unable to otherwise.
Valve man invents tongue and bum controllers [Eurogamer]