I'm late but I really want to have a changeling/skinwalker/mimic enemy, basically something which can surprise a player.
Imagine going through sewers and finding some scavenger/goon who acts normally and moves/shoots like a regular humanoid enemy but when you "kill" them some "The thing" warps/bursts out of their body and starts doing some crazy stuff.
Different scenario, you encounter a rathound or few, they act like normal rats but at som point they just stand up like humans do and transform in some kind of a weirwolf.
Or literal mimic scenario when some regular objects can be an enemy or hide an enemy within, like opening a crate but it has teeth or have some tentacle inside of it.
Or something between faceless and Callidus assasin from 40k, basically an enemy which can change/morph to get an advantage. For example having a nimble and hulking forms which have their own pros and cons, or throwing a flashbang and disguising themselves as nearby friendly NPC. Also lots of posibilites for quests.
