3 ideas.
Butt expansion. (which is a weird one for me since I more for breast and belly) and Path affect stats. Maybe as a option but I would think the paths the players can use affect the stat growth. belly/weight is defense. breast is mage, buff is attack, and butt is agility, not sure how a agility would work but maybe it would work like a dodge system.
group fights. while fighting one on one is ok it limits any aspects of team work. maybe you can have battles with multiple characters "in the field" and skills can do different things. like guarding with a shield lets that user tanks the attacks for the other teammates.