I think the OP means a tank that uses spells as a primary means of attack and enmity control/ generation.

There really isn't a precedent for this outside of Paladin, who used cures to generate threat in FFXI. Rune Fencer in FFXI was a melee based tank that was specifically designed to tank AGAINST magic attacks, but they still used their greatswords and melee abilities to generate threat.

I actually see Fencer as a class that uses Greatswords that could branch into Dark Knight and Rune Fencer pretty easily. Here's hoping =)

Red Mage could get tanky if they buffed Stoneskin or Protect or gave them another magic ability that increased armor so they had the mitigation even if they were wearing medium armor, and that ability also increased enmity. I think they'd be a GLD job with Flash also getting buffed to be their primary enmity generator, with WHM and BLM being required souls to have access to their cross class abilities.

ANYWAY, there are a ton of different possibilities. I'm happy just playing the game for now and waiting to see what crazy stuff SE comes up with next.