I'm thinking Dark Knight's rework will be more extensive than I initially thought.

[Jandor]

As far as dark arts goes, we can only speculate. But I personally would not mind having to relearn how to play the job as a result of an extensive rework to DA. I know DRKs trademark ability of sacrificing HP to boost damage isn't really plausible in FFXIV as a tank. If it was a DPS, this would not be a problem.

They could sort of do HP sacrifice on DRK tank.

Whenever you steal HP from an enemy it could duplicate the same amount in a job gauge, you could then use that stolen HP to boost damage or add special effects to skills.

It's not quite the classic 'cast from HP' but I think it would feel pretty good and fit thematically. The more HP you steal from enemies, the higher you can keep your own HP, the more you can boost your damage, boosted damage leads to stealing more HP from enemies, keeping your own HP topped up even more efficiently, round and round.