In order of likelihood:
1. Yuna (FFX): There's no way FFX won't receive a new character, and Yuna surpasses every other character in terms of popularity, story relevance, and gameplay potential. Should be an easy choice--though a "surprise" pick with Seymour or Auron isn't entirely impossible.
2. Shadow Lord (FFXI) - He's a pretty "generic" villain, and the size contrast between him and Shantotto would be hilarious.
3. Locke (FFVI) - I do not believe for one moment that FFVI would not be given a new character: it's one of the most popular games, and it has the largest, most diverse choice of characters in the series. With that said, of the many potential choices, Locke comes out ahead. He's the most popular character from the game, he was considered for the original, and he's pretty much always considered "second" to Terra. If it's not Locke, I wouldn't automatically go with Celes--someone like Setzer or Shadow would be a more unique choice.
4. Seifer (FFVIII), Gilgamesh (FFV), or "Badass Darth Vader-guy who was seen above" (FFXIV) - The last character will be a villain, but I'm not certain who. Seifer would make the most sense since he's fairly popular and was considered last time; Gilgamesh is popular too, but I think his viability is more questionable because he's not a "conventional" character. Using a villain from FFXIV would be a good way to promote the game, and it would give Lightning someone to oppose (like Shantotto/Gabranth in the original), but any character from that game might be too "new."
