I would love to see Miyamoto try his hand at 3D Sonic. It'd be interesting to see what he comes up with (Sonic in Super Mario Galaxy 2! Believe! )
Also, I would want him to do an Avatar video game. Seeing as Ubisoft had no idea what the heck they were getting into. *shakes fist at Ubisoft* Make being a Na'vi fun for Pete's sake.