I think that Black Mage would make for the best Square Enix rep. He's got the Nintendo Mascot character feel, like Pac-Man, Mega Man, and Sonic have, and as far as Final Fantasy reps go, he's specifically a classic FF rep, which is Nintendo HISTORY.
Finally, I have a rather unorthodox idea that would make Black Mage an even better Square Enix rep: BM should have a summon move. Much like Pac-Man, I think Black Mage should have some liberties taken with him to acknowledge other aspects of Square Enix representation. Black Mage may never have been able to do this before, but it's not a big stretch to give a mage this type of ability (like Pac-Man's Namco roulette and his Galaga beam grab attack, and like Mega-Man's moves from all the different robot masters).
Firstly, a summon special. This would randomly (or in a cycle) summon one of: Chocobo, Moogle, and Slime. Each would be cast out onto the battle field and have a unique effect, but only one could exist at a time. Don't worry about wasting one of his specials on this, there's plenty of room in his Smash attacks and other specials to encompass his many elemental and other attacks.
And going one step further, here's my idea for Black Mage's Final Smash:
Warp-
The Warp move was an instant kill attack that would take an enemy and transport them to a void in another dimension. So, I think this move should act like Mega Man/Duck Hunt/Captain Falcon Final Smashes, and when connecting with an opponent send them off screen into a cinematic. What's more, I think that the move could warp the opponent into a void where other Square Enix characters attack them! So imagine they get dropped into a black expanses. Roto/Loto/Erdrick appears and slams the opponent downward. Then, they are struck by lightning as CRONO lunges at them with a flurry of swipes from his katannas. Finally, the camera zooms in on a familiar face, and then zooms out as Cloud comes charging forward, finishing the player off with a swing of his massive sword.
Between these two moves (a summon for some of the cuter mascot characters and a cinematic final smash scene of the most iconic and more serious protagonist characters of Dragon Quest, Chrono Trigger, and Final Fantasy), the Black Mage becomes more than just a Final Fantasy rep, he becomes a Square Enix mascot.