I would make a music game! To use the motionsensors of the Wii controllers, and you should be able like make music! Like playing on air instruments, except with sound!
It should be called... uhm... Wee Rock! That's a good name! 
Aaahhh, seriously though, I think I would make a classic fighting game, like Tekken. I wonder why that sort of thing hasn't actually been done yet, the Wii controls seem like a good choice for some fighting







