League needs a new type of damage: Penetration Damage

Basically I was looking at the new keystone masteries and realized:

"Deathfire Touch and Thunderlords Decree seem a bit biased towards AP champions, since they can build penetration for them."

So that got me to thinking - what if League had a type of damage which switched between magic damage and physical damage, depending on which of the two would deal more damage to the enemy? This would mainly be beneficial to AD Assassins, since then their penetration builds would also affect their masteries for a period of time. Penetration damage could also be put on items such as Hextech Gunblade, to allow more versatility in the uses of them.

I just think that all keystones should be equally viable for both AD and AP builds, which is also why I think Warlords Bloodlust needs to be replaced completely.