I was thinking something like AD&D skeletons being weak to blunt weapons and resistant to pointy ones. It could add flavor to some specific foes.
This probably wouldn't be impossible. Certainly not a universal thing, but I could see some monsters having weapon-specific resists or weaknesses. Especially more esoteric critters.
Of course, it would probably end up meaning that there's a lot of monsters resistant to certain melee weapons. Maybe if we kept it minor (say, 10% either way) it wouldn't be too bad. If nothing else, it's half done because we've got the weapons (mostly) down.