It'd make sense to me if it wound up utilizing absorption as a main tanking mechanic, maybe you can use drains or something like Dread Spikes to create an HP buffer for yourself sort of like how Drain II functioned in FFXI.

Maybe have some passive abilities or self buffs that increase in effectiveness the lower the Drk gets in HP- that'd take some serious work to make viable though.

Also, I doubt it'll ever be a thing but I really liked the Dark Knight ability "See you in Hell!" From Bravely Default, take a big chunk out of an enemy's HP as a parting shot when you got killed.