I know your point by trying to bringing up DRK as Tank ... but i dont think another heavy armor melee will solve the lack of tanks ...
If SE don't want to use the best solution wich is: Tanking abilities on all melee with A Very Easy Aggro mechanic just like Ragnarok online 1 & 2 , where the lack of tanks doesnt exist.

Then i suggest the second option --> Blood Mage Tank a la Forsaken World Online.
Blood mage was a huge success there, Forsaken have way more tanks player than ff14 , because actually the game propose something we call "diversity" on their tanks choice.
If you dont want to add a Blood Mage, it could be Red mage or Blue Mage , with an original & different gameplay.
So If DRK fail at attracting more people to play Tank, i'd like to start in the futur an official survey to propose a Mage Tank, wich in my opinion would be more successful than DRK.
My Two cents !