This already exists infrequently. For example, wyverns and brigs each have a crit spot that is only revealed after you damage their front a certain amount. Cyclopes malfunction when damaged enough and start shooting around like crazy. Goblins and hobgoblins can have their heads shot off if you aim for their head, after which they’ll charge you recklessly and hobgoblins can’t use their shielding ability. My guess is they can’t extend this to most enemies because it’s a T rated game, so they can only have this functionality on mechanical enemies such as vex.