Yesterday we talked about what songs we want. Today let's talk about alternate costumes/color swaps that we want!
Disclaimer: I'm sure most characters won't get alternates, but it's fun to speculate.
As for confirmed characters:
Pit - Dark Pit is a given, considering he was based off of Brawl's black Pit costume.
Bowser - Dry Bones Bowser would be bad ass. We've already seen it in Project M, and seeing it in a mainline game would make me happy. Perhaps if his final smash is changed we could see a smaller Giga Bowser as well for a costume. As for something completely unrealistic, Super Mario 64 Bowser would be amazing and maybe convince me to get good as using Bowser.
Pikachu - Raichu? Seems very unlikely, but would be pretty cool I suppose. Sadly, shiny Pikachu just has lighter colored cheeks, so that's not really an option.
Fox - I can see Wolf being an alternate costume if he's taken out of the roster. That wouldn't make me happy as I loved Wolf in Brawl, but its a possibility.
Samus - I would've said Dark Samus, but that's been confirmed as an AT. Perhaps a Metroid Fusion costume could happen. I don't know if I'd use it, but I'm sure it would make a lot of others happy.
Link - Maybe a SS Link could happen. Dark Link was in Brawl, so that's probably returning. Pink-haired Link would be a nice addition as well.
Mega Man - Obviously the different colored suits will be represented. Nothing else that I can really think of here.
Villager - The obvious choice here is female villager. I'd be surprised if that didn't happen given Male WFT's existence.
Mario - Dr. Mario is another obvious choice. Maybe Metal Mario could happen given the fact that he's appeared in 2 Mario Kart games now.
Pikmin & Olimar - Maybe some of the other main characters of Pikmin could be represented? Seems unlikely to me, and I can't think of anything else.
Luigi - Dr. Luigi could make an appearance I suppose. Other than that I can't think of much.
Peach - Another obvious choice is Daisy as an alternate. Pink Gold Peach might happen, but that seems less likely to me than Metal Mario.
Toon Link - I'd love to see Toon Link in his outfit that he started with in Wind Waker. Hopefully we'll get his Spirit Tracks conductor outfit as well, although I don't know if that would work with carrying a sword around.
Sonic - Maybe Shadow? I hope not. Of all the characters who shouldn't get alternates, Sonic is up there. Let's not bring any of the countless, dumb characters from that series into Smash.
Marth - Assuming Robin gets picked as the FE: Awakening representative (which I think is more likely, personally), Chrom is an obvious choice here. Maybe Roy as well, but that seems much less likely. Lin would've been amazing, but sadly has been confirmed as an AT. Finally, this is incredibly unlikely, but perhaps Isaac?
Zelda - I wonder if SS Zelda would be able to work with Zelda's moveset...
Zero Suit Samus - I know including this would cause all sorts of unneeded controversy, but I wonder if Super Metroid's ZSS will be included.
Charizard - Shiny Charizard (Black) will certainly be included, right?
Greninja - Same as above.
Likely Veterans
Ganondorf - It probably wouldn't work out, but Toon Ganondorf would be great. His dimensions seem very different than the current Ganondorf, though.
Ness - Pajamas Ness would be great. It was one of Project M's better ideas.
That's about all I could think of off the top of my head.