Last summer a friend and I wanted to develop an iPhone game and came up with the following idea. Sad to say, we are both no iPhone developers and had no chance of realizing it. Nevertheless, I think the idea might be quite unique and maybe even cool enough to be actually implemented someday. (If you happen to be a developer and would like to do this, I would very much like you to contact me!)
Our concept was based on the idea of creating paper planes on your iPhone. When you start, you have a sheet of paper, which you can fold and turn by tapping on and tilting your iPhone in order to build your own paper plane model. Then you are able to paint it (e.g., by putting images on the sides and the wings).
The next step would be to let your paper plane fly (in 3D space, of course). You could on one hand throw it and let it fly on its own–e.g. in a virtual environment with special items like fans or walls–and measure the distance. On the other hand you could pilot your paper plane yourself and evade obstacles.
A final step would be to create an online platform to share your personal paper plane models and high-scores. Maybe later, you could even challenge a friend to play a multiplayer game with you (via WLAN or Bluetooth).