Lets talk Tenacity. That stat that's actually way better than everything else, except it comes out to less damage so it's not. It shaves off a tiny bit of Determination's damage bonus but adds in more healing and damage reduction. Much like current tanks, it's a bloated stat, an attempt to woo tanks away from Direct hit and Critical strike, but obviously it can't do this because nothing warrants it being useful other than blind bleeding edge progression.

Lets run with the assumption that the other sub-stats also get some rework or tunings, and lets talk about Tenacity and what it could be.

For one, lets get rid of it being More Det. (Though Det and Ten builds are pretty dumb. You won't set it any records, but lol at having effectively a second Darkside)

Tenacity currently:
Increases average damage at about 70-75% the ratio of other stats <= (Here's the problem)
Reduces damage taken.
Increases healing taken.

So lets do the reasonable thing and BURN IT TO THE GROUND.

Tenacity should make -tanking better-. Not "damage dealt", not "damage taken", but -tanking- overall. It should be the stat you work around breakpoints for before dipping into others. So how can we do that?

Idfk. Ya'll fickle. So lets try some things.

Tenacity does several things.
A) It -increases the potency- of offensives buffs the Tank applies to themselves.
B) It -increases the effective duration- of defensive and utility buffs the Tank applies to themselves.
C) It -reduces- the cost of actions.

So lets apply Tenacity to our Dark Knight discussed so far.

"Spinning Slash" applies "Whirling Step". This classifies as a utility buff. Whirling Step's effective duration (and thus overall use) increases with Tenacity.

A new Low-to-High Posture skill applies 100% parry chance for the next three incoming attacks. A defensive buff, thus the amount of attacks deflected increase, increasing its overall amount.

A Neutral-To-High posture skill grants the Dark Knight's next attack from High Posture to deal 30% more damage. An offensive buff, Tenacity would -increase- the 30% bonus, granting higher damage.

Blood Weaponskills would apply "Blood Weapon", a utility effect granting the Dark Knight MP when landing weaponskills for a few attacks. Tenacity is a dual bonus here - Resource reduction on Blood skills and increasing the amount of Blood Weapon charges per skill used.

In short, Tenacity serves to improve the -core functions of the tanking kit-. It would be the equivalent of Spellspeed's current state for Black Mage - Something you can tune around your own wants before moving to another sub-stat to focus.