True, with many franchises with so many owners, it can be complicated to implement. And that is without counting all the game design, programing, art, etc.... And balancing all these characters can be tricky.
Rocky would be an extremely complex case. I pictured him as a character that relies on many different counters based on the buttons you press while being hit(like bouncing off when hit, or reflecting part of the damage or stuff like that), instead of moving and attacking like normal fighters. Also, mirror matches would not work in this character`s case.