If people want a HP-sacrifice ability, why not give one that still supports the role of tank?

Like sacrifice 20% of HP to gain a 40% HP barrier? Still giving that HP sacrifice effect, but it's not actually contradicting the purpose of the tank.

Maybe a skill that doubles maximum HP but inflicts a bleed on the Dark Knight and prevents natural HP recovery? Having a huge HP gap for healers to fill in could make for an interesting skill.

Make the spell fencer job, and give them a darkness element. Then all the people who want their Dark DPS can just go there.