I'd say BB would transform through combos rather than gimmick button. There's really no other way to do it that works. It'd be nice to have complete control over what animal he transforms into at all times, but its simply too difficult from a developer perspective. I'd could see him having different stylized forms for his gimmick button. This is how it would work:
BB always has a default monkey/human hybrid form. If he performs certain actions, this will change what form he will change into without combos through his gimmick button. If he uses faster but weaker attacks, he will be able to turn into a cheetah/human hybrid form that has increased speed, but lower attack power. If he uses slower, but stronger attacks, he'll have a rhino/human hybrid form, which has decreased speed and higher attack power. These are mainly an aesthetic thing and he still performs his regular combos just with the stat boosts.