Tank role in the past mmos has usually been quite a bit support oriented and generally focused on making DPS'(and to a degree healers') jobs easier - keeping mobs off of them so they can focus on damage, positioning for comfortable backstabs and what have you and in pvp especially, tanks often are there to set up kills with cc and debuffs. My point was that even FFXIV, as watered down as tanking's been in the game, had some examples of that, therefore debuffs that increase rDPS are honestly more "in character" for the role than high potency nukes. The entire start of this direction of the convo was after all the conclusion that tanks should do more "tanky" things rather than get higher dps.
Personally I think that excessive treat dumps have been the main reason aggro kinda faded away in importance in this game. Because they allowed for tanks to largely ignore threat management, as long as dps and healers "pushed their free threat buttons", everyone got used to it being the right way - I mean it was the most optimal, so clearly game mechanics say it's right. Since we could usually hold aggro despite ignoring it and doing so resulted in higher dps - and therefore higher contribution to the party - any situation where it wasn't possible anymore just felt bad.
I think best way would be to bring back the old stance mechanics(not just for threat but also mitigation trade-offs), while keeping enmity management skills tank-only and make those sacrifice potency, but maybe not things like resource generation. Perhaps give dps and healers some emergency threat dump on very long cd or with some cost, so that they still have an option for bad tanks or deaths, but can't use them to trivialize the mechanic.
Shirk could also be changed to simply dump aggro rather than transfer, although I kind of like the idea of both tanks managing enmity together, regardless of who's MT/OT at the time.
I did kind of enjoy how post 4.3 DRK had 3-4 main threat management options with different levels of cost and strength, so you could decide which you wanted to use based on how badly the party screwed up on their enmity.
DA Plunge was something that you had to actively keep doing, but didn't actually cost you unless you needed MP for burst somewhere else.
Passenger was a bit of a loss due to dealing unaspected damage, but not too much and it provided a significant boost. You could also DA it for more damage and therefore enmity.
Lastly you had DA Power Slash, which costed you potency and resources lost from not using the other combo, but the treat modifier was pretty crazy.
Of course it didn't really come up all that often, but it was a nice "idea" that could've been expanded upon.