A sumo wrestler theme for a fighting game with weight gain would be very interesting and I would love to play something like that, but for this game I was thinking it could be something like Skullgirls, where the fighting style doesn’t need to make much sense. That way I can have more freedom designing the characters and their movesets.
As for training your fighter between tournaments, I really like this idea. The problem is that since I wanted to make 2D sprites for the characters, drawing different stages of fat for every animation of each character it could be very difficult. Perhaps this would fit better in a game with 3d models, but for the moment I’m not very good at modeling.