The problem with a DRK sacrificing its HP is the risk of dying when big hits come.
So what if, instead of having mitigation oGCD, he had HP refill ones ?

He could have skills like the current Thrill Of Battle or TBN, that could compensate for having lower HP most of the time, or even a skill to swap HP with someone that could also be used to give a good portion of HP back to the MT when you're the OT, or a Drain line of spells. And, instead of having GCD that would heal you (Like SoulEater, Clemency, Storm's Path, etc...), he would have GCD that would mitigate damage, so roughly the reverse concept of how tanks play right now.