Slaying sling bullets
Slaying sling bullets would make using slings more interesting
At the moment the only ammo with slaying capabilities are arrows and quarrels. Therefore, slings aren't very used by players. If there would exist slaying sling bullets (other than demon-slaying), specializing in slings would be a viable alternative, enriching the variety of the game.
EDIT: Stingray1 pointed out that demon-slaying ones already exist.