I have plenty of ideas for the next games, since they have covered most of the characters already. They need to have a bunch of what-ifs. Buu should have more absorbtions, like a piccolo-only one. He should have one for absorbing random characters, as well, like Frieza or Cell.
They need to have SS3 for Vegeta, as well as Golden Oozaru stages for both of them, so they can both then be SS4.
As for what you said about Super 13, it would be cool if we could see 14 and 15 in the game, too.