Yeah, my boyfriend was actually telling me about this and I thought it was a really interesting and good idea. I hope it's a balanced risk reward and isn't gimped like 2.0 WAR's self healing gimmick, though. IMO it should be sacrifice X% HP but have it work like Pox where it lowers your current HP entirely, and then everything you add on from there is a % of your new #.

So, if you have say 1000 HP as a DRK and you use Omen Shield, you bring yourself down to 900 HP. Using Blood Gain would bring you down to 360.

Obviously this doesn't work so great with the abilities you listed, but I think simply sacrificing HP that is easily healable might result in abilities being quite OP. I do like the idea of sacrificing HP for mitigation or special effects, though.