Change Sunsword to Sunshield, a shield of light (or shield of reflection in variants that lack a shield of light) which does not impede spellcasting when worn.

Every time Sunsword hits a monster, it should cause temporary blindness to the tune of several turns.

Alternatively, since that type of blindness may be tricky to implement, have it induce regular monster blindness upon a “critical hit”, with the same frequency as other artifacts like Vorpal Blade.

If a worm that walks is slain with Sunsword (and is thus disintegrated), it doesn’t dissolve into maggots like it ordinarily would.

Wielding Sunsword protects you from being blinded by an Archon’s radiance.

Merge Sunsword and Trollsbane like NetHack Fourk does (a single artifact that emits light, instakills gremlins and instapetrifies trolls, with bonuses to undead), except keep it a morning star and name it Sunflash (because a lot of these consolidated effects have nothing to do with trolls). Having it stay a morning star makes it an attractive choice for Priests, and raises the appeal of training morning star.

Make Sunsword out of gold.

Undead in Sunsword’s light radius will flee.

Anyone who wields Sunsword can perform the #turn command, which is flavored as brandishing the sword threateningly.

Give Sunsword an invoke effect that creates permanent light in a radius like the wand of light. “PRAISE THE SUN” optional.

Sunsword does extra damage to vampires and trolls, on top of its normal bonuses versus undead.