I think that we may not see any other 3rd party characters announced. However, we will see who likes Nintendo with the characters. Kojima's Snake is in only because Kojima wanted it to happen. And well, Nintendo and Konami never had any bad blood between them.
As for a Final Fantasy Character, well, that's an interesting idea but who would you pick?
I mean, Snake was Metal Gear Solid, just like there would be no Metroid without Samus.
Final Fantasy characters are for the most part one shot characters, and it is rare to have the game revolve around them. (At this point, someone is pointing out Ness and the pokemons, I am sure. However, Ness was the most important character in a game that was one of a kind for nintendo (how many RPGs has nintendo released anyway?). The pokemons were handpicked:
1 The anime mascot
2 A fan favorite (Jigglypuff)
3 A secret pokemon (who the hell got Mewtwo on their first run anyway?)
4 A young version of the mascot.
I mean, sure, they could pick Kain. But why not Cecil (the main character who could have a wicked transformation between dark knight and paladin)?
Terra? Yeah but what about Celes or Locke?
The best they could do for FF is simply make a cycle character like the trainer, and put it Warrior, Black Mage, Thief and White Mage. (That would kick ass if you ask me).