I think it'd be so close to the real thing that it wouldn't make any sense. why not just do the real instrument? what's appealing about GH and RB is that you can pick up a controller and instantly play Welcome to the Jungle. It would take hours and hours and hours to learn how to play that song. Even simple songs like a Weezer song would take hours of practice.
Ummm you would be picking up the real instrument and just plugging it into some sort of midi/usb convertor box. You could still use the instrument with an amp or whatever. It could have a training mode and a mode where you just jam along to songs. The whole point of the idea was too actually teach music theory to beginners.
those already exist. The point of rock band is to pretend to be a rock star without effort. Slightly related http://www.youtube.com/watch?v=mT4oBZJZhew Harmonix please
that would require a realistic guitar. You would need a separete guitar. It owuld not sell. And one of these "instructional" guitars is in itself a console for your TV and even has aguitar hero clone built itno it.
As someone who's been picking up guitar as they go, it's really worth it to go the extra mile and just learning the damn instrument. Rock Band drums are pretty similar to the real thing, though.
But that is not a game. That's the point, its a GAME. You liked duck hunt as a kid right? Why didn't you just go grab your BB gun and shoot ducks?