I've had an idea going on on my head for a while, and it would be awesome if this was in Smash 4...
You know how Wario has a "special costume" in Brawl, besides his regular look? He has both his Warioware outfit and his regular Mario series outfit. So I thought, why didn't any more characters get a special costume/s like that? I think that would be great, both to give you more looks and to sometimes even give you more characters without actually programming more characters (this would even be convenient for the developers!

).
Think about it. You know the Pokémon Trainer? Normally he looks like Red, but what if we could also make him look like Blue, or Gold, or Brendan, or May, or other trainers? Combine that with maybe giving the Pokémon the look of other Pokémon from the trainer's respective game (as long as it makes sense of course) and it's like you had a lot more characters!
Dr. Mario could return as a skin for Mario, Young Link could be a skin for Toon Link or whoever the heck's replacing him in this game (if missing, maybe he could be a skin for Link himself? I dunno, there would be issues there), Pichu as a skin for Pikachu, and so on... I think it would be great ^_^ What do you think?
*Flame Shield, 'cause you never know*