I know it's a far fetched idea, but think about this. The Wi controller has a speaker for making gun noises and all that. If it had a microphone included it could take advantage of your broadband connection and let you use it like a wireless telephone. A headset device would be even better, you could walk around the room chatting to other people who have headsets. Of course the Wii would have to come with a tiny bit of software that emulated the voice ip software we find on PC computers.
It would cost next to nothing and would appeal to everyone from school kids to adults.