What about creating an ACTUAL Dark Knight?

[MaraD_]

Option 1 is just changing the MP cost to an hp cost for The Blackest Night. Which I would love this change. Making sure to pop it at the right right time. None of this. "oops I missed the tank buster so I am going pop Blackest Night to cover for the healers." That would just ko us. Would bring a new level of play to the class.

Plus it feels more in line with the idea a DRK has to sacrifice defense, rather then their own offense. If you fail to pop TBN, you lose MP/offense. For this version u make TBN less effective, since it cost HP to begin with.

When I make my own content, and design classes with roles, I try to make them capable of mostly the same stuff, but they have things they "struggle" with, and things that are nearly passive for them.
For tanks its Mitigation, Threat, and DPS.
So I'd split the tanks as follows;
Tank:______Passive:____________Requires a lot of effort to equal the others:
TANK#1_____Threat is easy__________Mitigation is hard to maintain.
TANK#2_____Threat________________DPS
TANK#3_____DPS__________________Mitigation
TANK#4_____DPS__________________threat
TANK#5_____Mitigation______________DPS
TANK#6_____Mitigation______________threat
TANK#7_____Neutral on all 3.
(there's more combinations, but you get the idea)

DRK should be in the "Mitigation is what they have to work for, it doesnt come easily." category. Tank #1 or #3 (#3 if you look at how dark side used to work, and that WARs have always generated the most threat.)
Right now, all tanks have instant 1 button press mitigation they can fall back on. WAR was the closest, with storms path at one point having a -10% dmg taken at the end of a combo.
Then Inner Beast was part of a gauge they have to actively build up, but it came at the cost of DPS. (not the best way to handle that, but the closest thing FFXIV had to my example.)
This system would bring some variation based on what it is you have a preference for.
The current tanks generate threat passively w/o effort. And the current tanks have something between neutral and passive for mitigation, as it requires a button press, but hardly requires much effort, other than making CDs line up with tank busters.
DPSing as a tank is roughly neutral, with minimal effort to keep DPS up. (Though some might make some arguments based on which tank they find harder, such as the longer rotation of a PLD, or wanting to save MP for TBN/bursting TBN on DRK, which hurts DRKs DPS)