Now to give you some input. Try a puzzle game where you need to tilt and skew the phone or pad to reach the goals.
I like the idea of a tilting game, however without a real device to use to test my code with while developing it, I'm afraid that it might not work as expected if I just use the simulator to build and test the game.
Make it fun and polish the hell out of it.
What do you consider to be "fun" ?