That's a really interesting thought. We do still have issues where we get like Ken then Terry (or Mii Gunner then Mii Brawler) back to back, and for people who don't like that type of characters, its a bummer.
Each character having a list of groups that they belong to, then not allowing players to play a character in the same group consecutively would probably be a huge improvement. I would need to be careful to make sure too many characters aren't excluded, though. It would be tough to get right, but I think it would be really good.
I read that the devs didn't make this choice. Apparently Sony did.