Idk what Dark Knight you're playing but TBN is already a 15 second cooldown and it does not compete with the damage resource. If you use it correctly, it breaks and you get a free use of Edge/Flood of Shadow. If you use TBN without it breaking, that is a miscalculation on the player's part and also part of DRKs gameplay to use TBN correctly. But it could for example get a heal/regen after it breaks (or maybe if it doesnt, it heals instead vs mitigating incoming damage), to compete with the other tanks healing abilities. That would especially be helpful for healing team members, because DRK has no way to do that as of now. Abyssal Drain on 30 seconds would also bring it closer to Bloodwhetting levels of heal output
What I do like as "active" mitigation is something like Paladins Passage of Arms, because you have to kinda "aim" it at your teammates. Other mitigations are either directly targeted or just hit everyone in range. Maybe something like Healers Asylum, Sacred Soil or Earthly Star to place across the arena, too, for DR mitigation? Especially comparing with Earthly Star, if you could choose when to apply the healing/mitigation of a skill instead of waiting for it to either hit a threshold of HP or when the CD runs out like Heart of Corundum does.
As it is now, I definitely want all tanks to stay viable in similar ways, and not have any of them get turned into a joke for the sake of some gimmick that either has no real use or only fringe cases (that also have to be solvable by other tanks in case you dont have that *one* tank on hand, or would force people to bring that job to the encounter no matter what). Same as putting mitigation on GCDs, I would prefer if they all stayed mostly oGCDs as they are now. Stuff like Clemency is fine as a GCD I guess, but only because it's also kinda ridiculously strong as it is, and on demand as long as you have the MP.