I wouldn't worry about a 4th tank at a time where the current three are biting each other necks for what they don't have.

A disciple of magic "barrier" tank would be a nice idea. Converting damage done to shields or having some self-heals that add barriers (like adlo) with some unique resource. Some of its CDs would also add layers of HP similar to Thrill of Battle/Stoneskin and others reduce damage for when self-scaling is not high enough.
This will be different from the static % reduction the other tanks work with and would be self-scaling. But it also means with better gear you out-scale other tanks giving it a unique flavor in farm content.