I am hoping and praying that Jigglypuff is the Pokemon representative revealed. It would make sense (after Toon Link's reveal anyways). Not to mention Jigglypuff is the only character in Smash (aside from Pichu) that changes costumes as opposed to changing colors. This would be a sweet Halloween gimmick. If they changed up Jigglypuff even the slightest (by adding a fairy move) we would have plenty of pictures to document some cool changes.
I actually think they will want to hold off on Mewtwo to build some hype. They would probably reveal Mewtwo closer to the release of Smash 4 or maybe even keep him a secret to unlock or discover in the game. I think they did that with Lucario in Brawl (I could be wrong, I am going off memory). Mewtwo is a big reveal, he is probably one of the most anticipated reveals. Do you really think we would get it this early?
That being said, I believe Lucario or Pokemon Trainer are possibilities. However, if they are going to correlate the launch of X and Y with a character, I think we can count Lucario out. Lucario is definitely popular, and he is getting his new Mega Evolution. So he is relevant, but I just think that with the launch of such an iconic franchise that they would pick an iconic character. Not to mention this could confuse people. They might think that Mewtwo isn't going to happen if they reveal the one that is COMMONLY associated as the replacement of Mewtwo. It wouldn't be a clear and concise reveal.
So Pokemon Trainer is more likely to be revealed if we are going on relevance alone. Lucario has a Mega Evolution, but all of the Kanto starters do as well. But when I think about it, wouldn't Sakurai want to build some hype around Pokemon Trainer as well? Not only is he probably the MOST iconic. He is also the master representative of Pokemon. He is at the top of a lot of peoples "cut" lists just because of the sheer amount of development required. If the roster isn't finished yet, I don't think Pokemon Trainer will be finished by October. Not to mention that is A LOT to show off. 3 different characters in one? The amount of pictures he would need to take to make the reveal seem meaningful would be absurd.
However, a lot of people are speculating that a Nintendo Direct will be happening in October. So a trailer might be released. I think a trailer would be the best way to market Pokemon Trainer. Especially if they replace Red with a new trainer. So if we get a trailer in October, Pokemon Trainer is probably going to be the most likely. If we just get a picture reveal like the rest? That would be fairly lackluster.
That being said... I think they are trying to save the big and interesting reveals for later. Closer to the games launch. We seem to be getting the obvious reveals as of yet. Nothing to surprising. Toon Link surprised some. However, if people realized that Spirit Tracks Link was different from Wind Waker Link... it would not have come as a shock. The Wind Waker stage in Brawl and Toon Link's baton taunt should have been enough to indicate that they were indeed different Links.
What is the last obvious reveal we are waiting for? Jigglypuff. She is one of the original 12, part of the Smash franchise for 13 and a half years. We have already gotten 2 Mario reveals and a Zelda reveal. So Nintendo's second biggest franchise is overdue. We are most definitely going to get a Pokemon reveal in October. Jigglypuff is probably the most likely by principle of omission. I doubt Pokemon Trainer is finished yet. I think they are saving Mewtwo for later. Lucario wouldn't be a good way to represent the launch of X and Y. Jigglypuff being one of the most important Pokemon during Generation 1 and most of Generation 2 is a decent choice.
TL;DR
Jigglypuff