For those of you who want there to be alternate costumes, here's some good evidenceStart by asking, what is Masahiro working on? There's no story mode and less characters/stages then Brawl. Is he working on a battle damage system? Mostly likely not. Online multiplayer? Not extensively. This game will be bigger then anything the Wii could have handled, and Brawl had a ton of features. So what's he doing?
He claims there will be "Customization" to some degree. Why would said customization not be alternate costumes? For say he became tired of just color-swapping palettes.
Now for the evidence it may exist in the game:
1) We all saw the Male Wii Fit Trainer doing the exact same thing as the Female one, but he wasn't confirmed as a character. Why?
2) Characters like Link and Marth have been given their costumes from Twilight Princess and Shadow Dragon, while they have new designs in Skyward Sword and Awakening that weren't used for reasons unexplained. So, alternate costumes representing their games?
3) A piece of evidence not commonly noticed. Toon Link was in the game since the beginning. Why? He wasn't driving that train. He was facing the other way doing different poses, and there wasn't a steering wheel or anything sticking out for him to interact with. He was just standing there looking on to make it look like he was a part of the stage. And he was dressed in his conductor uniform, an alternate costume.
So, if all of this is correct, I really hope that this is a real mechanic, because it will really add to the game.