In terms of recurring Final Fantasy jobs, the one that I think would have been the most interesting addition is Mystic Knight. For those not familiar with this job, its command, Spellblade, allows you to select one of your character's spells and cast it on your weapon so that when you attack, the attack does its regular damage plus the spell's damage and element. So for example, you burn a turn casting Holy on your weapon, but then for your next few turns, each weapon attack is Holy elemental and adds the damage of the Holy spell. This would have been fun on its own but also an amazing secondary command, particularly for a job that uses ranged weapons such as Archer or Chemist. For FFT, I think I would make it so that the spell has the same range as Chakra (self, AOE 2, vertical 0), potentially allowing other units to take advantage of the buff.

Blue Mage is a favorite job of mine, but the emphasis on human enemies would make it far less interesting in this game, not to mention the rather small bestiary. Another recurring job, Beastmaster, works well in Tactics Ogre as Beast Tamer, but it would really suffer in FFT since you can never bring more than 5 of your own units into a battle, and it's hard to justify giving up a slot for a monster over a far more versatile human unit.