Redmage could work as a tank pretty easily. Different elemental stances than have various effects. For example fire stance you get Enfire and Fire Spikes. Enfire adds fire dmg and increase threat generation while Fire Spikes does a decent amount of damage to those that hit the mage. Water stance's Enwater could be for replenishing mp/tp while the spikes reflect less dmg but increase parry. Wind stance could be the dps stance with the En increasing attack speed and reduce threat while the spikes increase evade. Then you have other attacks that change based on what elemental stance you are in. Like Elemental Assault, if you are in fire stance you do a cone attack for decent threat but in earth stance you do maybe a stunning aoe centered on yourself.