Think about to make using defensives more rewarding. Mitigation or avoid/block/parry could create damage buffs and procs. Interrupting spells as a tank could have a chance to deflect them and send them back to the caster. Tanks could be able to disarm enemies for a few seconds which would add the enemy’s attack power to the tank for that period.
See how the shield in Doom - The Dark Ages works, this would be way more funny than the current way how tanks work. It is less passive defense and more revenge.