Couple options here. Viking, although it doesn't look like a caster, did have access to Thunder spells as at least one water skill (Tsunami) and could probably use ice abilities for defensive cooldowns and other visual effects as a means of distinguishing it from Warrior. Green Mage would qualify as a support-based tank that and also used hammers, which would knock out two of the requests in this thread.
Heck, if we had anything based around transformation effects, it would probably be a caster tank in this game.
