Honestly two to come to mind for me.
Terra is obviously the first. Beyond being a really cool character and the obvious quality of her game, I feel like Smash should really have someone that embodies the 16 bit and older RPG. Cloud is the major FF protagonist from the series boom with the release of VII and Hero is one that illustrates the entire history of Dragon Quest. But a dedicated fighter paying tribute to those early generations of Role Playing Games feels long overdue and while arguably both her and Crono have the right to that particular status, I think Terra just has that extra bit of character to her that feels like she belongs in Smash.
Vivi is the other possibility. While Terra speaks to one of the best games in the series' history, he is singularly one of its best single characters. A fantastic personal, great battle mechanics, and visually one of most distinctive appearances in Final Fantasy history. It both harkens back to the different mages in the franchise's history while still being just individualistic enough to make him stand out. And ironically while coming from the same console generation as Cloud, FFIX feels like such a contrast from VII, that, like Terra, Vivi would also be representative of a wholly different side of FF than him.
