If DRK were a job available from ARR, it definitely would be the lifesteal tank and WAR the heavy mitigation one, but it werent, so WAR got the lifesteal and PLD the mitigation back then... now DRK magic style of tanking can lend itself to both lifesteal and shielding and since TBN is a thing, that should be the direction they take us.
A direction i tought right now, quickly at that so there may be some serious flaw in my ideas, would be something like this:

get a "Brutal Shell"-like shield on our ST and AOE combos, change Dark Missionary to a 15% maxhp shield to the party, Dark Mind to a big shield and increase its cooldown to balance, and turn Oblation into a trait, giving raw mitigation to the shield we make.
Like this we could keep some light mitigation most of the time so our shields wont break instantly and Dark Missionary would became a very good party utility with a base 10% mit and some shielding, do this and we wouldnt even need much more sustain than Soul Eater and Abyssal Drain, maybe just changing the Dark Arts proc into a heal like the other tank spammables, just have to make sure the Oblation trait won't work with healer shields or it would became too strong.