For starters, look at the chart of Warcraft 3’s current armor and attack types, such as the one on the Warcraft 3 Liquipedia. Of the four armor classes that the vast majority of melee units have, piercing does bonus damage to 2 of the types, full damage to heavy armor, and 75% damage to medium armor. Piercing damage does reduced damage to fortified and heroic armor, but it doesn’t matter much. Fortified armor isn’t used by units, which means that you don’t need to attack it in order to effectively win the game. Piercing damage doing reduced damage to heroes is the one thing that keeps it from being completed busted, but it isn’t enough to stop ranged units from being extremely popular. Players with lots of piercing damage focus the enemy player units, kill them all, and then finish the heroes. The first player will also utilize their own heroes to kill the opposing heroes.
Now consider that most of the good fighting units with medium armor, such as Rifleman or Headhunters, happen to have piercing damage. There are utility units such as Spellbreakers and Raiders, that have medium armor and a different damage type, but because they’re are utility units, valued for their abilities, their raw stats aren’t good enough to compete with the rest of the units with medium armor. The result, if you want to fight an opponent spamming ranged units, you’re best option is usually to go with ranged units of your own. In order to lessen the overpopularity of ranged units, piercing damage needs to have a weakness that a different type of unit can exploit.
I believe piercing damage should deal 75% to heavy armor, instead of full damage. This makes piercing damage, used by the most popular units in the game, weak to heavy armor, which I believe is the weakest in the game. In the late game, this would lessen the effectiveness of ranged units, and make melee units a stronger answer to ranged units.
Tweaks will have to made to certain units with heavy armor so they don’t become oppressive. Towers typically have piercing damage and heavy armor, and with this change they would feel tankier because they’ll take reduced damage from piercing attacks. On the plus side, they will deal reduced damage to melee units, meaning they deal slightly less damage. Still, I think towers would need their hitpoints reduced by 20%. There might be other units that need tweaks, but Blizzard should make those decisions once they have a PTR up. In the long run, I believe this change would lead to more diverse armies in the meta and allow for more counterplay to ranged armies.