I LOVE the idea of a Tank class with a Mage Aesthetic.
but there are some limitations you have to work around.
1. They have to wear "Tank" gear. Which means their stats are going to be heavily based around existing armor stats and mechanics. So they have to have their offense based on STR, not INT.
2. They have to be in melee range. Being a ranged tank, and therefore being able to nullify close range attacks meant to challenge the tank is not really feasible given the mechanics in play for most of the game.
But I can still see it working.
I think Gravity Magic might be the key. You'd still need high VIT to work with High Gravity environments, so that gives you a good reason to have that sort of training. The gravity spells could act as barriers, preventing damage by forcing the attacks away from you, but you'd still need STR to stop the force from pushing you back (equal and opposite reaction). But Really, any sort of Barrier Mage concept would work as an Aesthetically 'Mage Tank' concept.
Surprisingly, this would lend itself to a "physical mitigation tank" in the vein of a WAR more than a Magic Defense tank like a Paladin.