I made a quick fantasy roster:
Obviously, some of those choices are more realistic than others.
I realize it's not the kind of selection that everybody wants to see ("No Isaac? Tingle?!"), but it's in line with the thinking behind Sakurai's previous rosters to me, anyway.
I know Sakurai said that Animal Crossing characters wouldn't work as fighters, but that was before the release of a feature film, City Folk, and an upcoming 3DS game. It's simply too big to ignore now.
Tingle and Goroh are the least likely to be included, but I have them pegged as semi-clones of Balloon Fighter and Takamaru, respectively. If the latter two make it in, I think Tingle and Goroh have a chance out of sheer convenience. Goroh isn't relevant anymore, but he
is the second most popular/recognizable F-Zero character, and Sakurai seems to like him; he appeared in Melee's FMV opening and he's an Assist Trophy in Brawl. Besides, there might be an F-Zero game in the works for the 3DS or Wii U
(I almost got through that without laughing.)
That brings me to another point. Sakurai might have knowledge of future releases that the public isn't privy to, so it's nearly impossible for any of us to make fully founded guesses about the roster.