I'm of the opinion that the only other viable Pokemon characters at this point are Mewtwo and Jigglypuff. Jigglypuff is a classic Smasher and can now represent the Fairy type. Mewtwo is arguably the most requested Smash 4 character, and its return was directly hinted at by Sakurai at the X and Y roundtable at E3. Assuming that we have lost both Ivysaur and Squirtle, I don't see why there is "only one spot left for Pokemon". Technically, Brawl had six Pokemon when there were only 4 Mario characters. Both Mewtwo and Jigglypuff are very viable characters in their own right; why can't we have both of them? They would both be perfect candidates for unlockable characters. I'm pretty sure that Mewtwo's return is why Lucario was revealed as early as he was; I think Mewtwo has taken Lucario's unlockable spot.
If there is a need for a grass starter for OCD reasons, I'd be fine with either Sceptile or Serperior (though, Snivy was in Greninja's trailer and will probably be a Pokeball Pokemon, so I don't think this is likely at all). Sceptile is the only Gen III starter that I actually like, and I can see it having some interesting moveset potential (Leaf Blade, Razor Leaf, Giga Drain, and so forth). Serperior could also have an interesting playstyle, since we haven't seen a playable snake in Smash before (lol inb4SolidSnakejoke).
Sylveon could also be pretty neat; I can picture it having Fairy type moves like Moonblast and using its ribbons for whipping, tethering, and grabbing opponents, like Ivysaur. But with Greninja already representing Gen VI, I don't see this as very plausible at this point.
TL;DR
I think the only other viable Pokemon characters are Mewtwo and Jigglypuff, and we will probably get both. Pokemon can have 6 characters in this game because Brawl technically had 6 Pokemon characters. Sceptile, Serperior and Sylveon would also be cool, but they aren't very likely.