The Main Character Mage would have to have something extra to him/her that would help balance the character out.

If it's a White Mage, they probably would have to have some strong weapon, like a Hammer (homage to FFI). Maybe give them a decent Hit%.

If it's a Black Mage, they might have a high evade stat or something (Lulu homage).

Making it some other type of mage like a Time or Blue Mage would be cool. In a sense Tidus was a Time Mage who focused on quick attacks.

Having a Blue Mage as the Main would be cool, because they could start racking up spells from the very beginning. I'd like for them to have the ability to syphon the spells (a la Kimahri's Lancet), as well.