Bring back old armor penetration item
Old armor penetration item doesn't caused issue in the meta and were not overpowered on ADC as they are on assassin. Remove duskblade or rework the item to something melee only . Every iteration of duskblade caused problems since it's release and still will if Riot keep try to add something who deal to much damage. We don't had armor penetration meta back the days, it was used by it's intended users and it was fair.
EDIT : Removal would be more of a fair choice , because as people stated armor penetration always cause problems. If people want it fair, let's remove both penetration, magic and physical and adjust champion that go on the weak side. I think it would make the game way more neutral in terms of balance. We keep % penetration for tanky foe but remove flat penetration.