I agree with SSBF. I think that Mewtwo and Zoroark are by far the two most likely options from existing Pokemon, with any others only having an outside chance, and only assuming Sakurai hasn't already picked a rep.
I do think however, while perhaps not the most likely situation, a 6th gen Pokemon rep might be more of a possibility than some think. I'd wager that by now Game Freak is working on the 6th gen in some form, and if there is a Lucario/Zoroark equivalent, it would probably be one of the first Pokemon finalized, as it will be used early and extensively in promotion. I don't think it would be too much of a stretch if Game Freak approached Sakurai to include this character (or vice-versa), as around the same time it would be unveiled for Smash (assuming it would be unlockable like Mewtwo/Lucario) it would probably coincide fairly well to when the sixth gen is right in the midst of promotion/release (late 2014/2015). Even if SSB4 releases before the sixth gen is fully revealed, it is probable this Pokemon would already have decent exposure and popularity.
There are alot of ifs, like if Game Freak is far enough in development around now to suggest this character, if the character would work in Smash, and if Sakurai has enough to work with this early. Still, I think the timing would coincide in such a way that it would be doubly beneficial, and something at least one side might look in to. Of course if this happened, it would be very unlikely Zoroark (or any 5th gen) would be included. That said, at this time I still think Zoroark is the more likely character, but I'm not ready to discount the real possibility of a 6th gen.