Thats sounds pretty awful, this games class diversity is already so watered down they may as well all be the same class what you are suggesting would make classes even less specific.
If im a black mage I only want to be able to cast black magic, no more do I want to hear you can heal because you are a conjurer.
So what I would like to see is something like:-
CON - White Mage = Can only cast party buffs and healing magic
CON - Black Mage = Can only cast Destructive and DOT magic.
THM - Red Mage = Can only cast Healing and enfeebling magic
THM - Star Mage/Oracle = Can only Cast Astral/Umbral Destruction and DOT magic.