So generally in mmo's its normally a holy trinity.
Tank, dps, healer.

Why not add something else like a synergist (buffer) or sabatuer (debuffer)
Or even both into one class? You know for stuff like haste, shell, silence, blind ect.

It would make the gameplay feel more Final Fantasy.
It would help keep the genre true to its nature and diversify itself from other mmos.

Closest thing we have now is a bard.
I'd love to see one of these classes, I know Time Mage has been suggested before.