Dark Knights have always been a risk/reward type of class. Sacrifice HP to do obscene amounts of damage. This is the theme I hope they stick to.

If I had say over how it was though.

DPS focused tank.
Most attacks would burn HP to do more damage. (and probably some kinda self buff to make all your attacks burn HP for more damage)
Higher over all defense but a much lower HP pool then the other tanks.
Status effects would lower defense ratings and accuracy.