There are also plenty ways to do HP sacrifice, but a bit like TouchandFeel implies, these all require underlying changes to both combat, class and encounter design paradigms. We can't have any of them while the dev team insists on this copy&paste class formula which, granted, helps due to the large amount of classes this game has to keep around and continuously add to.

That being said, I feel my favorite variant of this would ultimately be something akin to the Unholy Armor where you sacrifice X% current HP to gain <hp-lost> * Y% (>100%) as a shield with no duration.