(or vice versa) I don't think it would be broken because even if someone built this against an all AP team, a player on the AP team could still improvise a sub-optimal physical damage build and get a lot of value from the physical damage boost.
An item like this would allow builds previously reserved for only the most toxic of trolling to become legitimately viable across all elos. Could be a fun addition.
Then people would buy just that, and every other slot would go to the opposite.
Cutting an entire damage type in half, on top of any Armor/Magic Resistance/Damage Reduction you already have, sounds broken.
In current day League of Legends, a 5 man AD/AP team is indeed a disadvantage, as the enemy team only needs to buy Armor/MR, but at least you can build Armor/Magic Penetration items to counter that.
But adding this item into the game? Then a 5 man AD/AP team is an instant loss. You can't itemize against damage reduction.
And no, you can't just build AD on a Mage or AP on a Marksman and expect it to work. That's wishful thinking.
I don't really mind
I 
a Small inch of MR on a tank makes me do 0 dmg so I don't even need to send my skillshots anyway
