Naaah. I don't particularly like that idea. It might be okay for an addon to it or something, but not in the base game.
Besides, they're only using stuff from Dragonball Z, not Dragonball or Dragonball GT. The whole Max Power Kaméhaméha thing was from Dragonball, never even flashed back to during DBZ.
Plus, they say that they won't add any more characters.
Furthermore, (running out of ways to start sentences...) Master Roshi wouldn't be a good idea. At no point during the series was he ever in a position to fight Krillin, let alone Majin Buu or even Frieza. I don't like the idea of it. He never even learned to fly!
Perhaps, if they added Mr. Satan, and someone pretty much remodelled/reskinned that, it could be alright in an addon. My verdict - DO NOT ADD IN BASE GAME.