From a DRK perspective:
TBN:
This skill should be a consistent dps gain when the shield breaks. It should reimburse both the cost and give 50 blood. Then change it to its PvP description (i.e. 20% for self or target party member, so that your teammate shield will only break under a substantial attack. This isn't something that should be haphazardly spammed in the hope that it breaks on a random auto. The counterattacks should be your reward for knowing all of the cleave timings.
Bloodspiller:
DRK offers the least in the way of personal burst. If so, we should offer more consistent dps on average. (This still works out to be harder than playing a burst-focused class in the long run, because any loss in uptime is costly, rather than just during your burst window).
If Bloodspiller is to hit less hard, then it should at least hit as often (if not more frequently) as equivalents on other tanks. This could be done by increasing the blood gain on Souleater to 20, as it is in PvP. Speaking of Souleater and PvP, the life-steal should apply in Grit, as it does there.
oGCDs:
In 3.x, DA usage on oGCDs was built around the fact that you could apply DA on an earlier combo action and apply it on a later oGCD. Now that DA procs off of most GCD actions, it forces an unnecessary double weave. The cost should be built into the ability. Likewise, DP needs to be reworked so that it sees more use. Just make the DA effect baseline.
In the long-term, I would like to see C+S cost blood and return MP, and DP cost MP and return blood, but I'm not sure if that will be feasible in this expansion, given the timing of when we learn certain skills.
Also, if this tier has taught us anything, Plunge should be 20y, like every other gap closer.
Blood Weapon/Price:
At a 40 second recast, BW doesn't have good synergy with raid buffs. It should be 30 seconds, and be usable in Grit. Blood Price is useless in single target and a bit overpowered in AoE, and should either be re-tuned or re-purposed.
Defensive Cooldowns:
The easiest way to solve DRK's defensive issues is to shorten the recasts on both Shadow Wall and Living Dead, probably by a minute each to bring it in line with WAR. They could and probably should increase the recast on Dark Mind if they do this (or rework the skill entirely).
Sole Survivor probably could do with a rework as well. It's a very niche skill with limited use. It should be a HP/MP leech effect while active.
Grit Activation:
In the interviews leading up to SB, the devs pointed out that one of the main sources of imbalance in HW was the flexibility with which WAR can stance dance compared to the other two tanks, which is why gauge costs were implemented at the start of SB. This was universally disliked. If we can agree that tank stance activation costs are awful, then no tank should have them, period.
In particular, there is no reason why Grit or Darkside should have MP costs. There is a GCD penalty in place already. Having the double penalty on DRK is silly. In fact, the MP cost no longer exists in PvP.
Summary
TBN: Creates a barrier around self or other party member that absorbs damage totaling 20% of your maximum HP. Increases Blood Gauge by 50 and restores MP cost when full 20% is absorbed.
Souleater: Increases blood gauge by 20 and restores HP outside of Grit (50% absorbed).
Bloodspiller: Restores own MP (1200 MP).
Carve and Spit: 450 potency at baseline, costs 2400 MP (DA cost is baseline).
Dark Passenger: 240 potency/mob at baseline, costs 2400 MP.
Plunge: Increase range to 20y. Increased enmity generation.
Blood Weapon: Reduce recast to 30s. Make blood weapon usable in Grit.
Shadow Wall: Reduce recast to 120s.
Living Dead: Reduce recast to 180s.
Sole Survivor: Absorbs HP/MP over time while active.
Grit and Darkside: Remove MP activation cost.
Increase the MP pool, to allow more room to float your resources.
A number of these changes already exist in PvP. Just port them over, and they'll work wonders to the class.