On the topic of utility -
Dark Knight's utility at the moment probably amounts to simply being the tank that loses the least from being in its tank stance (read: *being* in tank stance, not from the act of turning it on) due to Bloodspiller, and I'd say that's pretty much it. Its a good MT from a raid DPS standpoint, I guess.
As someone pointed out in another thread, TBN qualifying as utility is a very big stretch considering that at no time is a 5K shield necessary beyond what cooldowns the target is already likely to have running, and it also tends to be inferior in the long term to things like Apoc and Palisade, in addition to risking a DPS loss in comparison. So DRK definitely needs something else in this regard. And you can't give DPS-based utility to one tank without giving it to all of them lest you want to go back to huge discrepancies in personal DPS like those we had in HW.
At this point, since DRK's identity is kind of in the dumpster, I wouldn't be opposed to looking at some previous FF and FF-related games' lore for DRK abilities, without any of the HP sacrificing nonsense that would hurt DRK as a tank in this game.
Akin to sacrificing HP, DRK could get something of a defensive equivalent to Wildfire, that compiles all of the damage you take for, say, 15s, and then converts a percentage of the total into a raid-wide shield at the end of the duration. In this sense the DRK is indirectly sacrificing HP to benefit the group.
A possible Living Dead rework could be something similar to Last Resort or Life/Death from Bravely Default, wherein Walking Dead is simply cleansed by an Esuna equivalent, and during Walking Dead you get an added benefit of increase damage dealt/reduced damage taken.
Why we don't have some sort of self heal that costs mana/blood escapes me, as we are a tank, after all, and our self-healing is by far the weakest, and yet we do not have the ability to convert this one resource into HP even though half of our job is converting blood into mana, mana into blood, or either into potency. Seems we should be able to convert at least one of those resources to HP.
Given the nature of our mana as a resource, there's actually a lot of inspiration from BLM that I wish SE would take for DRK. A reverse Convert (sacrifice 20% of maximum MP to restore 30% of maximum HP) feels like a no brainer for DRK, as do abilities like Manaward and the old Manawall. Maybe something like the latter for ourselves, to shore up our physical mitigation, and the former to be cast on party members instead.