If there's any "general FF rep" I think it's going to be Onion Knight who can class change into black night/sage or Bartz Klauser with some Black Mage spells and animations.
Otherwise, I don't think the black mage class is by itself big enough for Square Enix to justify him. Hell, they'd probably go for Vivi just so at least there's a name attached to him.
But as far as Final Fantasy goes, I think they're done with that. If anything I could see Square Enix really pushing Nier (specifically Automata and 2B) or Star Ocean.
Or maybe even Kingdom Hearts.