I think a neat way to have balanced yet highly magically advanced melee weapons would be to have them require a power source.
Of course, you could go for the ol' life force consuming, body and mind-altering evil weapon, but I think I think there are more interesting alternatives out there
One such alternative could have the weapon's special magical ability effectively require some form of energy crystal thing which is loaded into a weapon's body before the crystal and it's energy are consumed upon activation of the weapon's ability.
Perhaps these crystals could also be attuned to different elements, or could contain the spirits of animals offering or both offering different ways to attack.
Oh and it doesn't necessarily have to be a crystal, could be a piece of wood, or metal, or bone with energy magically etched into it.
Or perhaps someone could just shove any old magical artificial (assuming it's small enough, or can be altered to be small enough) into the weapon to power it, though this would also destroy the artifact.