I think they should just make a Natal Fit.
Id really like some kind of virtual "Personal Trainer". Natal should be able to detect dumbbells, a skipping rope, or whatever you want to mess with and integrate it into your Training routine.
Also it could easily measure your progress, eg how many sit ups / push ups / .... you can do etc and kinda motivate you to do more by giving some feedback.
IMO such a game would probably sell as well as wii fit if it's well done and ms could attract some more casuals.