Maybe to add a bit of spice to the melee system, why not add a special animation to finish off an opponent. It can be performed from behind a full health enemy if you successfully sneak from behind, or performed anywhere if the enemy is at critical health, then you'll will try to approach closer to perform the execution. If it seems far fetched in multiplayer, then consider it in the single-player against enemy A.I.s.
https://imgur.com/6Ig2if6