The Longbow of Diana should create arrows on-the-fly as a passive effect when it is fired, rather than creating a stock of +0 arrows as an invoke ability. The arrows could be ordinary arrows with an enchantment equal to the Longbow’s, but it’s probably more interesting to clone arrows from your quiver (assuming the thing you have quivered is suitable to be fired from a bow), as this lets you keep multishot bonuses from using same-race ammo or use silver arrows and preserves the incentive to have well-enchanted ammo. Cloned arrows should always be destroyed when they hit the ground, so you can’t use it to create hundreds of highly-enchanted arrows.

A slightly different way to take this idea would be that arrows don’t get created on the fly, but automatically return to your quiver when they hit the floor. This means that any arrows that get eroded by hitting a rust monster or something will return to you eroded, and any arrows that get mulched will still be lost, preserving the ammo management aspect of the ranged combat game. On the other hand, it could be argued that removing ammo management for the player is a great benefit for a quest artifact to have.

Valkyries should be able to guarantee Mjollnir auto-returns safely (with no chance of hitting you) either by being piously aligned or having maximum Luck.

If you have an autoreturning artifact wielded, and nothing quivered, using the ‘f’ command will throw the artifact, without making you have to use the t command then select its inventory letter.

Artifact shuriken The Silver Star: deals +d8 damage to all targets. Like Mjollnir, it returns to the player’s inventory after being thrown, but doesn’t have any chance of hitting the player. Since there is only one of it, it cannot be multishot.

This would be a good first sacrifice gift for Monks.

Arrows fired from The Longbow of Diana will never break and will automatically return to you. It also counts as your racial bow regardless of your race.

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

Artifact arrow that auto-returns to your pack and cannot break. But there is only one of it, so it can’t be multishot.