If you've got a decent physics engine, make a platformer. We need one that handles half decently and runs on the PC.
Physics is pretty easy. I don't know why people can't get that down right. Maybe I just have a more powerful language to use? Anyway, a platformer could be good. Maybe one with online multiplayer and a level editor. Heck, maybe the host could even send the level to the clients, and the clients could choose to download it after they beat it... We will have to wait and see. I'm still working on the character creator tool. Almost done, except the rotation seems to be a little off haha.
EDIT: I just fixed the rotation, just going to add some more features, then I'll post some screens.