For Mewtwo, I would say he has a 60% chance of getting in. It's not especially great, but he was intended to be in Brawl, and on of the Melee requests list, he's tied with James Bond as the most wanted character who didn't appear in Brawl.
King K. Rool is pretty much the same, 60%. He hasn't had a major video game appearance since Donkey Kong 64, which was more than a decade ago. That being said, he has in appeared in a number of spin-offs, his franchise was revived not too long ago, and he has a decent amount of popularity.