Question is what would fit as a tank though.
People have mentioned ninja only because it ended up tank in Xi. It was not intended that way nor does it make sens lore wise.
Red mage wad mentioned. Rdm is a jack of all trade type so might be possible but in return its offence would be nerfed drastically which might puss off a great deal of fanboys.
Dark knight wears heavy armor but preys on its own life for increased damage. Souleater is one of its core abilities, a drk is supposed to e very risk reward class bit suitable to tanking either.
I can't really think of anything suitable atm other than ruining some more beautiful jobs like they did with summoner.