Make the Longbow of Diana able to change shape between the different types of launchers (bow, crossbow, sling), either by invoking it or simply by applying it. This allows Rangers to play with whatever type of ranged combat style they prefer, and allows gnomish Rangers to use a crossbow without being coerced into switching to bow later.

Characters with at least Basic in sling can do something (use a touchstone? flint stone? any gray stone?) to break rocks into slingshot, which weigh 1 and have comparable to-hit and damage to flint stones. Slingshot may be weapon-class instead of rock-class. The higher the sling skill, the more shot that one can get out of a single rock. There should also be rare silver rocks, which can be broken into silver shot.

The Heart of Ahriman returns to your quiver when slung. Or at least add some returning sling-ammo artifact.

Increase the damage of slung touchstones. Thrown luckstones retain their lowish damage but have a chance of getting “lucky” hits that do much more damage or instakill monsters.