DRK changes that would make it really fun to play

[Archwizard]

Unfortunately, if you define DRK as draining HP to deal damage, then that really isn't going to work well when it is a tank

However, that's not actually how I defined it. You'll note I said "use of health as a resource" and "becoming more powerful as health gets lower", both of which can broadly fit the Dark Knight archetype without necessarily having to literalize "drain your HP to deal damage". Hell, damage doesn't even have to come into it.
For instance, you could have DRK gain skills that increase in effect the lower their HP is when they use it, or based on the damage they receive in a given period -- self-healing along the lines of Essential Dignity or Macrocosmos for instance, or a burst of damage reduction that snapshots their missing HP, giving more mitigation the lower their HP is (like the Hellblade's Last Resort from the Bravely series).
Alternately, you could rework Living Dead to be a Full Immunity skill that drains a percentage of HP over time in lieu of the "all or nothing" Doom clock.
Since 14 emphasizes Barriers in DRK's kit, you could have them convert some percentage of HP into a barrier that raises their effective health, or add a cooldown that fills in all of their missing health with a barrier while giving them life leech until the barrier breaks. The possibilities are pretty much endless.

I previously wrote up a skill idea for DRK I called "Martyr", intended to be its sort of Passage of Arms. It would let the DRK channel a high potency barrier, but at the cost of transferring part of the damage taken by nearby allies to the DRK.
I played around with its MP consumption to ensure it would be best used as a group mitigation rather than just an extra strength TBN, but that's not really relevant to this topic; the point is that it uses the DRK's health pool as a resource for the whole raid.

And even if we do bring damage into it, you can also see in the PVP version of DRK that it plays with the "drain HP to deal damage" concept without necessarily needing to invoke it in a way that compromises the DRK's survival. Shadowbringer can be used at the cost of HP, but this cost is negated by Dark Arts, effectively doubling the survival value of TBN.
It actually wouldn't take that much effort to clone something like that into PVE, and would make a very effective way to remove TBN from MP (should the need ever arise in some rework).

Combine any number of ideas along these lines and you could create a tank whose entire kit is based on Risk vs Reward.
But right now the only RvR skill in DRK's kit is Living Dead. And speaking as a healer... a lot of people still misunderstand the "Risk" element of it.

[Daeriion_Aeradiir]

And even if we do bring damage into it, you can also see in the PVP version of DRK that it plays with the "drain HP to deal damage" concept without necessarily needing to invoke it in a way that compromises the DRK's survival. Shadowbringer can be used at the cost of HP, but this cost is negated by Dark Arts, effectively doubling the survival value of TBN.
It actually wouldn't take that much effort to clone something like that into PVE, and would make a very effective way to remove TBN from MP (should the need ever arise in some rework).

Combine any number of ideas along these lines and you could create a tank whose entire kit is based on Risk vs Reward.
But right now the only RvR skill in DRK's kit is Living Dead. And speaking as a healer... a lot of people still misunderstand the "Risk" element of it.

The PvP kit works because healing is intrinsically a limited resource in PvP, and 90% of the healing is controlled by the DRK itself, leading to the DRK having majority control of its own gameplay flow.

in PvE, such a kit would be ridiculously hard to balance, dps wise. Do you balance for the fact healers can theoretically vomit tons of heals into the DRK to continuously let them use higher power potency skills with no risk the entire time for greater overall damage than if the healer was spending those GCDs to dps, or do you balance around an expected number of uses of those skills per minute assuming healers aren't healing more than necessary? Because in the former, you make the DRK kit severely underpowered if your healer doesn't want to play personal healbot, and in the later, severely overpowered if your healers are willing to play healbot. Not to mention the arguments that will no doubt arise in DF/PF from DRKs demanding personal healbots in the care of the latter. This entire balancing methodology is precisely why the devs made old darkside prevent MP infusion from outside sources, since mana transfer skills/songs could be used to inadvertently give DRK more power than the devs wanted it to have. That way, with MP locked exclusively to the DRK's actions, the system became entirely self contained.

There's also major problems with any tank that wants to be low on HP for any reason, since it literally runs counter to the healer role's modus operandi. Making its mit be based on how low or high your HP is completely robs control of your own ability to use said mit in an effective manner, since it robs personal control from you based on what the healers do (In fact, this is precisely one of the reasons old living dead was reviled so much - you had zero control over it as a mitigation tool and were entirely at the healer's mercy). If there's a mechanic like solo Ahk Morn towers or taking Cauterize w/o invuln, or taking a double buster solo which requires incredibly high levels of mit to survive, having one or more of you defensive options fall into the range of 'if your HP wasn't low enough when you used them, you auto die to this mechanic because the mitigation on them is now too weak to survive the hit(s) even at full HP post heal' just because you got an infusion of cure 1 + tetragrammaton right before you used your mit button(s), then you just kneecap DRK's survivability and make its defensive kit wildly inconsistent.

Even beyond that, there's just the issue of how you'd balance the actual mitigation values contrasted to the other tanks. Since you either end up in a situation where DRK's mit is weaker than every other tanks 90% of the time and only equal if the DRK is at low HP to prevent its mitigation from being overpowered, resulting in a tank that is far more cumbersome to utilize in a group setting compared to just picking literally any other one, or you make their mits equal in 90% of situations to maintain parity with other tanks, but then make them OP when the DRK is at lower levels oh HP, resulting in DRK being wildly out of whack and a mess to balance since it would easily trivialize tougher mitigation checks or basically have infinite invulns with proper setup.

There's literally only one case in the DRK kit where lower HP could be a conditional flair, and that's its personal healing. Having soul eater & Abyssal Drain have modular lifesteal based on missing HP wouldn't break anything, so long as the potencies are capped off at reasonable levels.

It's why MP is the perfect substitute to keep the general flow of 'resource bar go up and down rapidly, but you can potentially put yourself in a bad situation by overspending' that the HP gimmick of DRKs in past games utilized on. It's a self contained system that the DRK possesses full control over, and doesn't run counter to both the tank & healer role's entire modus operandi.

TL : DR: The PVP kit works because of the environment its designed in. There's far too many additional factors in PvE for such a kit to work without having massive balancing issues, positive or negative. For Health scaling damage/mitigation to work in PvE, the differences between minimum & maximum %s/potencies/etc would have to be so small to maintain parity with other tanks in 95% of situations that you'd simply run into the issue of 'why even bother in the first place?'

[TouchandFeel]

I think that one of the main factors against making HP a resource for DRK that seems to be getting overlooked is simply that it is not encapsulated solely within the control of the DRK themselves.

Damage buffs that players can place on others are very highly controlled, existing within the 2-minute raid buffs, and are that way for the reason of being predictable and more easily measured in terms of the expected output gains which is necessary for proper balancing between jobs and the tuning of content. We can of course debate if 2 minutes is too long of an interval of buff-based burst cycles and whether it should be shorter/longer or stuff like that, but the importance of such buffs being defined to exist within very controlled parameters of availability and frequency is undeniable.

By making HP a resource for DRK to fuel ability usage, you are essentially breaking that above tenet because HP is far more fluid and something other jobs, particularly healers, can at any time and constantly be pumping their resources and effort into, in function allowing healers to pretty much give or deny DRK a dps buff whenever they please. Now the DRK is no longer in full control of their own performance on a moment to moment basis and in order to maximize their individual output, has to rely on other players focusing on them and providing them a constant external source of a resource for them to use. That will just lead to a very frustrating situation. Frustrating for the DRK because they won't feel like they are in control of their own capabilities and game-play, frustrating for the healers because they will be expected to go above and beyond to be constantly feeding the DRK so that they can perform better and immensely frustrating for the developers because this would be incredibly difficult to balance around due to the drastically increased variability of DRK dps performance resulting from this forced interplay.

It's not too hard to come up with various ideas about how to make HP a resource on DRK and it's not too hard to come up with justifications for why your version is good or works, however the sheer level and amount of complications and issues that come with trying to implement this will always make it a hard sell.