Like mana based regenfighting (higly poisonous warlord, well built gorgon with cydstepp + mystic balance), lifesteal based regenfighting - seriously, if blood hunger didn't erode resistances this would be one of the most broken thing ever - (dracul pala).
I also used to believe protection stacking didn't exist, but EM with platemail + taurog's shield can yield great results. Have you tried a panzerrouge? Give an orc rouge these stuff with RBS and regenfight whatever you want. Entanglement will make you 1st strike as well

Oh, I found something that may fit in the "hole". Max HP decreaseing. Orb of Zot does it, and nothing else, and it doesn't quite combo with anything in the game yet. But I could imagine it being an ability of a class, who removed X max HP/hit/level, and that would consume blackspace nicely if you overused it. But I'm probably off with that.