What would a new tank even contribute at this point

[Alexandre_Noireau]

The issue with a tenacity related tank is that they would need to make the damage stats you get from it comparable to Crit or Det.
Unless they change the current combat that is. Because right now having more tenacity would be pointless since nothing hits hard enough to warrant having it. The tanks who do more damage would always be favored.

Tanks should have access to "Retaliation Damage" giving many skills/abilities or spells (All mitigation basically), including a Role Ability this type of damage: Make Tenacity (Role-Based Damage) govern that damage with more weight than all others - literally inverse for those actions the damage stats with tenacity as it works now.
Also they could modify Tenacity to weight more just for Auto-Attack.

You don't buff Tenacity to increase all damage, keep it as it is for that - But increase Tank actions oriented damage and a new damage type entirely for it's own to govern.

[Eastwall]

dark knight is best in frontlines, the pve version needed nerfed as to not show favouritism

I would replace "best" with " completely busted"