I think there are already visual hints about enemy body part situation (visual bleeding), and the idea of research needed to be able to see “numeric” HP is quite great!
Not being able to know how much HP does an enemy have after being shot by a soldier an not knowing how much bleeding damage are they going to get would increase the inmersion and dificulty of the game.
players would need to use up will points overwatching “just in case” more often at the start, as the soldiers (and the player as their tactical commander) are not familiar with the enemies they encounter, so they do have a reason to start panicking.
Also, the posibility of enemies “playing dead”… thats genius! The start will be scary as F… because of sudden ambushes from enemies you know nothing of. And combined with the enemy adapting their strategies to the player style, then maybe they should stop playing dead shortly after they realize that their enemies somehow are not failing anymore from that trick, and maybe develop other stealth methods