just some ideas i've had to perhaps add some more depth to gameplay and raise skill ceiling.
Subsonic ammo - quieter (no supersonic crack) but lower effective range, travels slower than normal rounds( would require more skill at longer ranges), could have less penetrating power than normal rounds, although the low penetration should perhaps be only applied to objects not players and body armour for gameplay reasons. cost 1 or 0 or wutever the devs feel like.
Locking doors - ppl could shoot out the lock of a closed door to "lock it", requiring enemy players to either breach of shoot out the hinges, thus revealing their location, basically a door with the lock shot out cant be opened in the standard way, and requires some more thinking.
"Death throes" - this is already kinda ingame but just as an aesthetic thing. so when u shoot and kill someone in anywhere but the head they drop on the floor and kinda wriggle around a bit before dying. i propose that when this happens the ragdoll is shouting, calling for help, saying their hit etc until they acc die. mechanics could be add where the player can shoot the downed ragdoll to finish the kill and stop the shouting. this can be used in tactical gameplay to not alert other enemies and encourage well placed headshots.
welp there they are, if need be i could explain further. hopefully these are fairly simple and don't require any new art assets just the fleshing out of existing aspects of the game for add depth and fun