Unfortunately, I doubt we'd ever see a legit Final Fantasy Warriors game, due to them being competing companies, but I wouldn't put the idea of SE making a similar IP for themselves. In any case, based on the games I've played, and who I want (versus who I think would actually make it in):

Rydia: Powerful Summoner and Black Mage. One of the most iconic characters in the series. Also has a GREAT outfit, and is one of the very few characters who uses a whip in the series; she'd definitely be in place for some amazing character animations. Special attack would be summoning the Mist Dragon. Ice.
Red XIII: Love the dude, top favorite from the game. Gotta have one non-humanoid character, and like Rydia, he'd be set up for some awesome character animations. Especially any special attacks he'd use--potentially one that's similar to Meteor, but instead is calling down the moon. Fire.
Quina: Such a weird dude. Not a favorite, but definitely unique. Special attack would be its tongue wrapping around a small perimeter of enemies, then using that group's attack on a larger radius of enemies. Water/Earth.
Rikku (FFX): One of my favorite characters in the entire series. Would be our nice punchy girl, her special attack wouldn't be Mix, but pulling a large gun (reminiscent of the canon boss at Lake Makalania) and shooting in a line AoE in front of her. Lightning.
Fran: Fan-service. Earth.
Aymeric: You know I have to shout out to my boy. Your basic all-arounder. Holy/Light.

Bonus characters/special unlockables:
Beatrix: Just absolutely broken Shock attack. Holy/Light.
Kuja: Primarily magic user, special attack is Trance Kuja with a circular AoE. Dark.


I'm sure I could come up with more, and this was actually incredibly fun to imagine!