Sorcery should get a flat magic pen rune similar to Ravenous Hunter

Something like...

"Some random rune name" grants 2 magic penetration plus an additional 1 magic penetration for each unique champion takedown.

Right now there are champions who are severely lacking in flat magic pen from not being able to take sorc shoes(Cassiopeia) or champions preferring to take something like merc treads or boots of swiftness. The only way to get flat magic pen outside of shoes is Liandry's torment while AD have a number of different options for both lethality and flat/bonus armor penetration.