I'm not a big Dark Knight or Tank player, but I did level all jobs and occasionally return to it so consider this a very casual view of the job.

DRK to me felt and still feels like I'm just pressing my buttons for the sake of it. I'm not building towards anything, nor looking forward to anything. It has an "active" phase but it's basically just a bunch of cooldowns being up at the same time. The fix doesn't need to be (in fact it probably shouldn't be) "here's a 1 minute CD rotation" but I don't have enough experience with the job to say what it should be, other than "I like how PvP DRK uses HP for stuff, I'd like to see that".

One random idea is some form of mitigation (perhaps a TBN rework) who's potency scales depending on how much HP you've spent before casting it. I don't know how well that would play though.

But yeah, I don't know, sorry if this isn't helpful. DRK just doesn't give me the impression that it has an identity. The other tanks don't have that issue, Warrior feels like a brute, Gunbreaker like a skilled merc, Paladin like a holy protector. Dark Knight looks edgy, but playing doesn't make me feel edgy at all. Living Dead does more to the job's identity than the rest of the kit, you could recolor the skill VFX to red and call it Flame Templar and could probably get away with it.