Theoretically speaking, we already have this, it's just mainly used on Objects.
I've always hoped to see them find a way to apply it to NPCs or even player costume parts. However I don't expect to see the last mainly because we continually heal from low to full in seconds.
"Null is as much an argument "for removing the cottage rule" as the moon being round is for buying tennis shoes." -Memphis Bill
I like to use combat auras and it gave one idea to me.
How about low health auras for some NPC's? Some giant monsters would look great when their auras activates at low health. I'm thinking about steamjets, sparks, cluncer etc. Would be great feeling to see they are almost down not only for health bar but some graphical effects also.
Prunejuice is warriors drink.