Dark Knight's current problems are that it is boring and repetitive to play, consisting largely of hammering the same button between every GCD while repeating the only DPS combo over and over again with no alternative combo ender, let alone an upkept buff or dot, with fewer and lesser defensive options than the other two tanks, little to no self-sustain, and very little opportunity for burst or any sort of interesting or fun optimization of its damage. These problems call not just for potency changes, but a revision to its entire playstyle and toolkit to bring it somewhat more in line with how it was in HW, but still retaining its current blood gauge system in some form. A lot of people have said that the primary thing they like about DRK is the speed of the job, so that should be made front and center in a more interesting way than "press this same oGCD button over and over again before almost every GCD". My primary proposals are making the speed granted by Blood Weapon a central part of the job that is always there, not just during Blood Weapon's duration, and making the blood gauge simpler and more user-friendly as well as fueling solely offensive oGCDs.
Here's my proposed changes:
Dark Arts
MP cost increased by 50%
Siphon Strike
Remove Dark Arts effect
Souleater
Remove Dark Arts effect, remove HP restoration, now gives a 10% attack speed buff that lasts 30s (so it would be used for every fourth combo, like Storm's Eye), also increases blood by 20
Bloodspiller
Now combos off of Siphon Strike, restores HP regardless of stance, also increases blood by 20, retains Dark Arts effect, potency decreased as it will be used a lot more
Power Slash
Now increases blood by 20
Dark Passenger
Remove Dark Arts effect, now single target, cooldown reduced to 1s, MP cost removed, potency changed to be the same as the Dark Arts bonus potency (currently 140), now costs 50 blood to use
Quietus
Remove Dark Arts effect, now an oGCD weaponskill with a cooldown of 1s, still costs 50 blood to use and regens MP per hit as before, potency nerfed to be less than Dark Passenger
Delirium
Remove old effect, remove blood cost, now increases the potency of blood gauge actions and Dark Arts bonus potency for GCD actions by 100%, duration of 12s
Blood Weapon
Remove attack speed boost, now increases blood by 10 for every weaponskill executed under its effect instead of the prior blood gain effect, still restores MP per hit
Blood Price
Now increases blood by 10 for every instance of damage taken under its effect instead of the prior blood gain effect, still restores MP per instance of damage taken
Sole Survivor
Now additionally has a Dark Arts effect: grants a large HP regen over its duration
Grit
MP cost reduced or removed, additional oGCD action added that turns it off (the action to turn it on would still be a GCD though)
Also change potencies across the board to keep DRK's overall damage with the Souleater speed buff similar to how its damage currently is without it
These changes would make DRK play faster than it currently is outside of Blood Weapon, and would also make it more dynamic, with more depth to gameplay than just hammering Dark Arts every GCD with only a single DPS combo. The resource management of blood would also be a lot simpler and less messy in terms of numbers than it currently is, increasing at easily predictable intervals in consistent amounts, with spending blood gauge being similar to SAM, albeit a whole lot simpler. It would also be burstier than it currently is, while not being as ridiculous in its damage distribution as WAR. The Sole Survivor HoT as well as Bloodspiller absorbing HP regardless of stance (similarly to Storm's Path) would improve self-sustain. Adding an oGCD action that turns Grit off would remove the current danger of double-clicking it back on by accident when trying to deactivate it, as well as making it much less clunky and weird to deactivate. Reducing the MP cost of Grit would also make it easier to regain enmity and improve defenses in dungeons if you run out of MP during a big pull while it's off or forget to put it back up after a boss.
Another thing I would want is for Dark Arts to be reworked slightly so instead of immediately consuming MP to give you a buff that then modifies the next action you use with a Dark Arts effect, it instead gives you a buff that makes the next action you use with a Dark Arts effect consume MP then to give you that effect. So like, instead of the way it currently works (you use Dark Arts, MP is immediately consumed, next action with a Dark Arts effect is buffed), it would consume MP later at the time the action with a Dark Arts effect is used. The reason for this is so that the ability does not become usable midway through a GCD, but is always available at the start of a GCD, preventing the clipping that commonly happens after Siphon Strike when the MP doesn't show up until halfway through your GCD and then using Dark Arts clips your next GCD because you couldn't use it before the MP showed up. This would potentially create issues with wasted MP if the player is near cap though, so another potential fix to this problem (and probably a better fix) would be to drastically reduce the animation lock of Dark Arts, or remove it entirely like NIN mudras.