If you're looking for useful features to build I would find a well being tracker very useful. Exercising isn't my problem. Injuries are. If the bot would manage my schedule so if my back is stiff it recommends certain exercises or if I haven't done cardio in a while it recommends that instead of weights etc it would be fantastic. With ML I can imagine being able to build a statistical model off the back of this that would be able to perfectly curate a program for optimal health based on frequency of exercise, type of exercise and reported well-being.