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.