While it's a good idea, don't count on people banning them from tournaments (right off the bat, at least).
Maybe as a separate mode where you alter each characters' stats as you please. You know.. there's Stamina Mode, there's Slow Mo mode, etc. We could have a Level-up mode where you spend coins, or ability points to rearrange each character's status, NOT affecting the main multiplayer experience.
Otherwise, like I said, these uber characters will be the new MKs and they won't be stopped after like 3-4 years when the community FINALLY comes to an agreement that they are indeed broken... And even THEN, not all smash communities everywhere will agree to ban them.
So... just as an unlockable extra feature for me. It would be fun though. Making Charizard super fast, though not particularly strong, or Bowser being able to jump as high as Falco.
Edit: I'm thinking of something like a user-defined Brawl -... MAYBE, if you could upgrade every character, then it could give a whole new depth to tourney play... But only if executed correctly. This would be really hard to balance IMO.