It depends on the focus of your game, but i really like when FPS games adds mechanics that doesn't involve just shooting at the enemy, but rather shooting at unique weak-spots that rewards the player even more, like shooting the back of an enemy deals more damage – or an enemy has two heads and you need to damage both of them to stun/kill – or even a non-enemy related puzzle that you need to aim at something to solve. Basically making more use of the gun to progress in a fun and diegetic way!