For me, outside of the new controller to fit my hand properly (read my other ' if' style post), I would say I would invent a voice recognition unit to accept in-game verbal commands.

This would help in playing squad-based games, FPS' s (like Halo/Halo2), RPG' s, and racing games in the fact that you could communicate with NPC or other team members verbally, rather than having to push and/or click your way through commands.

Other games could benefit from this technology, but the four genres I mentioned seem most likely to benefit.

I know that similar technology exists, but it' s not being used on any of the current console systems.