Also keep in mind the game balance. It's been planned and Balanced with the characters towards DBZ powerlevels. That means ssj3 tops. If they add another level to certain saiyan characters, that would give them another incentive to play them, and even more powerful in the long run, and Vegeta and Goku will already probably be some of the most powerful characters when topped out, so that last level would be pretty insane.
Adding movie characters, such as Brolly, would be kept at similar powerlevels to other characters, and would be much easier to adjust balance to then giving a couple characters additional forms, and is probably much more likely to be included.