I don't like the idea of Mage tanking, BUT, if you want to try something new, you can think in a specialization of summoning.
My Wizard in DnD once helped the group as a OFF TANK with a summon and some buffs. You can think in some spec that you can summon something like a Golem that you control and work around that with improvements for better tanking.
As an off tank ok, but I don't like the idea of a Mage stealing the thunder of a Warrior, Paladin, Black Knight, etc.
this game have a witch and a necromancer, we have plenty of sumoners i think
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El juego tiene nigromante y bruja, ya tenemos bastantes invocadores creo.