Tank : Titan/Sephirot
WHM : Moggle
SCH : Phoenix
AST : Valefor? (not yet released, but as been mention in the game by gagarua)
Monk : Ifrit
Dragoon : Leviathan
Nin : Ravana
Bard : Garuda
MCH : Bismark
Black Mage : Ramuh
SMN : Bahamut


That's my idea, Tank sharing titan/sephirot is mostly due to limitation with how many primals or elder primals we've encountered so far.