I think that there was a thread already for this but I forgot my answer so I may as well post my ideas in here.

The original Black Mage, because the game seemed to lack a male mage, and there weren't many mages to begin with in that game...

I would like to have had Celes because again she was part mage, but she also focused with some attacks, and her runic ability could be pretty cool

Locke because then him and Zidane could have an epic "thief" battle and he was a main character from VI, sadly I can't think of another reason.

Maybe split Cecil's two classes, and make Kain the chaos side for the Dark knight because there wasn't enough to focus on either class I found, and it would be cool to see Kain in the game...

Maybe Sabin because he had all his blitz moves that could be changed into his abilities, and it would be cool to use a fist fighting character.

I just realized, with this I must sound like an FFVI fanboy -__-'