A DRK Rework Suggestion for 7.0 (or beyond)

[Aluja89]

That's because DRK came in HW, but I personally still wouldn't make it a lifesteal tank. Simply because that was never in DRK's identity in FF. It did usually have a drain spell in its dark magic spell list but that's about it.

DRK needs to be a magic shield tank, TBN is what it's known for so it should expand on that, like a personal Haima. Change Darkmind into something that absorbs blood with every GCD into its shield and for HP Abyssal Drain should be a spell with cast time, single target 800P heal with splash damage+healing. Finally, Souleater gives a 200P shield and 200P heal.

[Undeadfire]

DRK needs to be a magic shield tank, TBN is what it's known for so it should expand on that, like a personal Haima.

It was, and it dominated the role during Alexander Savage, and it was horrible to balance.

Barriers are a lot more powerful when it's stacked with mitigations, barriers strengths saves a lot of healing, and enables more DPS outputs, SE won't ever do this. Double Barrier Healers have always dominated the raiding scene because of the sheer amount of mitigations. Currently in EW it's Ast with Sge/Sch since Ast has the most broken healing ability Macrocosmos.

[Archwizard]

I personally still wouldn't make it a lifesteal tank. Simply because that was never in DRK's identity in FF. It did usually have a drain spell in its dark magic spell list but that's about it.

That's more because Dark Knight was never meant to be a tank in past FF games, not because of the presence or importance of lifesteal.
It wouldn't do to have a tank who sacrificed most of their HP dealing damage, after all.
Historically though, absorbing the life force of opponents was the only self-sufficient way Dark Knight had to recharge Darkside. XIV is the only case in the series to have Blood Weapon not even act as a healing effect, though this is probably as an artifact of once having an MP-consuming Drain spell.

Meanwhile if we are going on series precedent as you have, there's a sheer lack of barrier effects for Dark Knight due to that usually falling under White Magic, which is Paladin's domain. "TBN is what it's known for", and even its anti-magic bent, is unique to XIV.

[Shurrikhan]

It wouldn't do to have a tank who sacrificed most of their HP dealing damage, after all.

So long as it can make up for the HP spent, that'd actually do fine. It'd be another thing that could go wrong (like mistiming CDs), but equally another thing that could go right (additional burst damage before damage lulls or ways to improve healing-received efficiency during damage lulls, and/or a way to more granularly prepare for damage spikes -- essentially banking HP during lulls for later burst damage taken).

though this is probably as an artifact of once having an MP-consuming Drain spell.

I have to agree. Such made BW far less susceptible to drift, as you could build resource at almost any time and every form of potency afforded by MP at least had some sustain value attached. Back then, it was a smart choice. It just stands out now that we can't spend MP on Sustain... which, honestly, your AD/Bloodspiller would rectify, if not for the slight damage loss of using them when you need healing rather than solely during raid buffs (which brings us right back to the old HW DA issue of maximizing direct potency over rDPS-when-accounting-for-healer-GCDs-saved... though that also comes down to healers having been largely changed since then to have very little GCD healing).

[Archwizard]

So long as it can make up for the HP spent, that'd actually do fine. It'd be another thing that could go wrong (like mistiming CDs), but equally another thing that could go right (additional burst damage before damage lulls or ways to improve healing-received efficiency during damage lulls, and/or a way to more granularly prepare for damage spikes -- essentially banking HP during lulls for later burst damage taken).

Oh don't get me wrong, I've thought about it (like having an X% of remaining HP cost on Delirium), or even compromising with effects like a "incoming healing is negated up to X% health" debuff.
(I really wanted this to be the flavor of TBN in particular, to sell this idea that the DRK cannot normally toss out barrier effects and is shielding you at the cost of his own health, which would also create further incentive to use Spikes unless offtanking.)

Trouble is, whether it's for damage or defense, this leaves you in a really awkward position as either the main tank (especially on Savage raids where autoattacks hit as hard as a tankbuster in Normal, or during W2W pulls where you're already melting) or when tasked with splitting damage with the other tank -- and either way continues the trend of being the hardest tank to heal, unless you have enough self-healing to counter those effects... which would then make you easily the hardest tank to kill if you chose to ignore them for survival purposes. And I don't want to create another Bloodwhetting WAR.
Literally no middle ground so long as the healing demands are relevant, you're either a ragdoll or a juggernaut, which makes it a nightmare to balance. And if the demands are irrelevant, they would exist purely for flavor, and be on the chopping block for being unnecessary.

So I just focused on the element of "The DRK already sacrifices his HP in the course of being repeatedly lashed by his enemies." Mitigation skills give bigger MP returns (yay Blood Price precedent) and Living Dead gets easier to deal with, depending on the damage you were fed by the enemy.

Not to mention that would go against the "pragmatism" of this design. Much as I enjoy the idea of risk-reward based on overcoming self-inflicted penalties (just look at Martyr), such double-edged swords should be relatively rare or outright situational so that they have minimal impact on encounter design or overall tank balance, rather than be central to the playstyle like... oh, gambling whether or not to stack mitigation on current TBN so as not to waste damage?

[Aluja89]

That's more because Dark Knight was never meant to be a tank in past FF games, not because of the presence or importance of lifesteal.
It wouldn't do to have a tank who sacrificed most of their HP dealing damage, after all.
Historically though, absorbing the life force of opponents was the only self-sufficient way Dark Knight had to recharge Darkside. XIV is the only case in the series to have Blood Weapon not even act as a healing effect, though this is probably as an artifact of once having an MP-consuming Drain spell.

Meanwhile if we are going on series precedent as you have, there's a sheer lack of barrier effects for Dark Knight due to that usually falling under White Magic, which is Paladin's domain. "TBN is what it's known for", and even its anti-magic bent, is unique to XIV.

First of all, you're not understanding the tank fantasy. Being a "tank" has nothing to with how their abilities work but how they look with their equipment and what sort of role they play in combat. Tanks are those that look like the biggest threat and fight at the forefront, the reason tanks deal less damage is because tanks are the ones that engage with their opponents head-on clashing with their weapons while everyone else pokes from behind.

But, you're also wrong on the barrier and general tank stuff. There are a few cases where something akin to it is employed.

Bravely Default/Second:
Adversity:
For three turns, your P.Atk, M.Atk, P.Def, and M.Def all rise by 10% each time you accumulate damage equal to at least 25% of you maximum HP.
*The upper limit for P.Atk, M.Atk, P.Def, and M.Def is 150%.

Absorb Magic:
Triggers at the start of the turn.
Nullify damage from magic attacks during that turn and restore your MP by the amount of MP used to attack you.

Life or Death:
Your P.Atk, M.Atk, P.Def, and M.Def each rise 50%, but you suffer doom status.

Bravely Default 2:
Strength in Adversity:
Every time you take 25% HP damage, attack, magic attack, defense and magic defense increases by 10% for 3 turns.

Last Resort:
Physical and magical attack power increase as HP is reduced.

FFX-2+Last Mission:
They can have passives for nullifying debuffs/ailments.