Tank forum is already being flooded with these and obviously the last time I made a thread like this it was "too OP" so I had the brilliant idea of trying again but trying to make sure Dark Knight would still be fairly balanced.
Since Darkside is what caused a lot of imbalance do to it's changes and needing to spam more Dark Arts for raw potency boosts and The Blackest Night make it obvious that you would rather take a different tank do to how all of the math needed just to even make MP usage even work, so simply; put Dark Arts is now a trait which more or less functions the same as it does now but it also enables Dark Passenger and Dark Mind outside of Darkside, which means more MP for the Blackest Night but increases MP and TP cost for those potency increases while Darkside is active (yes potency increases can only be obtained while Darkside is active).
Power Slash certainly sounds and looks like Power Slash should, but the numbers say other wise, and with the above mentioned change to Dark Arts, the potency (and combo potency by extension) would go up, instead of just more aggro generation, and what should the combo potency be, I hear you ask, 650, you know, make it feel like Power Slash.
Of course the monotony of spamming 1 of 2 combos still persist but not as bad with the above mentioned Power Slash change, and people really want Scourge back, but we need to keep with the theme of sustaining damage and add sustaining utility, (think pre-Stormblood WAR) Scourge would need to be combo action(combo action to get effects is Hard Slash by the way) for this to work, a DoT that has the added effect built into the DoT itself of increase physical damage recieved by 5% and reduces physical damage dealt by 5%, but since this is a unique debuff if you were in a 24/8 player raid or had 2 DRKs in your party because Duty Finder, it would reset the duration already applied effect of the first DRK that applied it, and it has the same potency as Syphon Strike.
Now to bring the level 50 weapon skill that got reworked into an oGCD ability, call it Debilitate and combos off of Scourge but is the magic equivalent of Scourge, and has the same potency as Souleater.
Now for the current Delirium, it needs to double blood gauge generation of Blood Weapon AND Blood Price, but only after you get the Enhanced Black Blood trait.
Speaking of Blood Weapon and Blood Price, Grit restriction on those abilities need to be removed and Blood Price should be the magic equivalent to Blood Weapon(think PLD Fight or Flight and Requiescat).
Also Grit would need another effect to justify damage penalty, basically Bloodbath that DPS somehow need but DRK doesn't even get because they need it more, but now get with Grit buff(I would stay in tank stance just for this buff alone).
Don't worry, Souleater would also have the HP abosrb effect no longer locked behind Grit... HP absorb would get a lot stronger if Grit is active( because of Grit change)
Sole Survivor is another ability that is generally useless, but let's make it so that it restores 10% HP and MP should Another Victim status fades before the target dies.
And finally, we have The Blackest Night... shield on party member effect is PLDs, thing delete that effect, and make HP shield give 25 blood gauge if it does not break.
BUT we all know none of this will ever happen...