DRK changes that would make it really fun to play

[Archwizard]

in PvE, such a kit would be ridiculously hard to balance, dps wise.

I think you're misinterpreting the point I was trying to make.

I'm not saying "we need a 1:1 copy of the PVP Shadowbringer" because that would be ridiculous since, as you pointed out, PVP has an entirely different scope and context than PVE. A DRK dies in PVP and only he suffers; a DRK dies in PVE and your group just lost their tank. The PVP version of Shadowbringer is spammable as long as you have the resources, making it available significantly more often than you can use the accompanying TBN and allowing you to dump HP until you're at a critical level. Obviously, we can't afford something that high risk.

Rather, the point I was trying to make was focusing on the use of Dark Arts to subvert that cost, negating any such risk for a tank in PVE under normal circumstances. Presently, the existence of Dark Arts doesn't "reward" proper use of TBN, but rather refunds the cost TBN had in the first place in order to induce a "counterattack" narrative via Edge/Flood.

Were something like PVP Shadowbringer to be introduced in PVE, tuning knobs such as a cooldown and charges could be set in place to ensure that it's only used roughly as often as you can acquire Dark Arts, with the raw HP cost merely providing you an alternative to casting it when you don't get DA so that your mitigation and DPS won't negatively impact one another. Same flavor of Dark Arts we have now, but it actually rewards you rather than just refunding.
In an extreme (and however unlikely) case, you may even use such an HP cost as sort of a magic wand to charge up some of the other effects I previously listed, giving you a small measure of personal control over your HP as TouchandFeel noted we lack. I'm not trying to advocate this route specifically, I'm just noting the open possibility to make and connect such tools.

Overall though, the reason I brought it up at all was to highlight that DRK's history and flavor provide plenty of opportunities for creative tanking tools (especially with PLD intentionally being straightforward and WAR not having any thematic magic or gadgetry), which is part of what makes its current position so underwhelming.

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.

God if I don't know it from DRKs screaming at me to stop healing them 'cuz they popped LD at the top of a pull with two HoTs, an external DR and a barrier on them.
(Like, I don't know what you want me to do there, bud. I'm on SGE, if I sneeze too hard it heals you. Learn how to use your invuln.)

Of course, I'm also convinced the 6.1 LD buff was just a holdover for a future rework, since they did the same thing for skills like Acceleration or the entirety of MNK's kit last expansion before bigger reworks in EW. The literal layers of complication on it now (in lieu of just... removing the Doom timer) have that certain vibe, y'know?

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.

And I also suggested that a Macrocosmos-esque effect that charges up as you take damage would fit too, rather than just snapshotting while you pray you hit the mit before Bene goes off.
... And even that couldn't be any worse than Oblation.

The list I threw out wasn't comprehensive, nor end-all-be-all. At the least, it was my attempt to illustrate the versatility of the theme besides just "Dark Wave go brrrr"; at most, an effort to get creative juices flowing.