This isn't really a game, but it could be quite useful to some folks.
Overview
Natal's full body motion tracking is perfectly suited for exercising. Pretty much any exercising technique can be implemented using Natal. Couple this with the infrastructure that MS built for Primetime and you are looking at a way to get a unique routine on a daily basis. Every day will have set times where professional trainers perform routines live. Customers can tune in and perform the exercises with the trainers. There is a performance index that display how well the person is following along. People can party up with others and socialize(a big reason for going to the gym). Since Natal has a built in mic, no need for the headset. Since Natal can track four people, four people can simultaneously be following the same routine. If you miss the Live session, it and the past two week's sessions can be streamed on demand. There can be a number of different trainers available for different exercise techniques.