Not just retro characters, but retro character that haven't had a follow up or sequel in over a decade. The Ice Climbers only game was Ice Climber in 1983 (as well as the game "Nintendo Vs.", which was a compilation of several Nintendo NES games for multiplayer purpose). Pit's only 2 games came out in 1986 and 1991, the latter only being released in Europe and North America (until a Virtual Console release).
That is why I think the retro character won't be someone like Little Mac (who has been recently revived). Instead, look for more obscure characters. However, Little Mac could be the new yet old or "previously revived" character. Remember, Snake was revived for MGS1 after a fairly long hiatus after Metal Gear 2. The same was true with Zelda, not appearing in a Zelda game for about half a decade.
Still, I don't know if Sheik/Snake could also be counted as "WTF?" but eh, whatever. I'll let someone else come up with those such theories.
Yeah, that is the kind of retro character I meant too.
I think if Sakurai doesn't view Little Mac as a retro character anymore, the retro character he does decide to reveal will be much harder to predict, although if the first trailer does make its debut in Japan in the fall instead of E3, I think Takamaru would be the best retro to pick, as he would get much more recognition there. Of course the retro character could also be someone like Mach Rider or Balloon Fighter (a pipe dream of mine, lol), although I suspect those are less likely.
Personally, I always viewed Snake's category as Third Party, instead of as a revived or WTF character (although I did go WTF when he was revealed,
). I think (and hope) that Megaman will take this place in the trailer.
I think the characters that would best fit in the mega-popular newcomers category would be better off as unlockables, such as Ridley, K. Rool, or Mewtwo, however seeing any of these in the trailer would have the same effect (actually a bigger effect) than seeing MK or Wario. I'm not sure who would be next in line as the mega-popular choice after these three, if hypothetically they indeed were unlockables.
For the newer character, I think a number of characters would work well here, Isaac, Shulk, Palutena (who technically isn't new, but barely had any recognition before Uprising), Mii, or even Miyamoto's new IP character (if the game is revealed before SSB4 is).
Hey, don't be afraid to choose different characters (there's dozens of fairly forgotten retro characters to choose from) or make a different theory (based on various factors).
Actually during the process of writing this out, it's given me even more to think about, so my characters might not be that similar to yours after all. lol
Anyways, I'd love to keep talking about this, but I nearly fell asleep writing this all out, so I'm out for now. I'll be back after I get some smash-filled dreams.