Game Mechanic idea: Null Damage.

Simply put, it'd be a sub category to the current Physical, and Magical damage. It could include many differences from the primary types but the main thing would be that this damage type is not affected by penetration. The idea is that it'd be a primary damage type on champions that are otherwise not full damage champions, like bruisers, and tanks.

By "Sub category" I mean it still does the exact same thing as the damage type it's linked to. Example: An ability does physical null damage so it'd still do damage coded as physical and interact with stats like armor, but it wouldn't benefit from penetration stats.

This could add alot of creative balance to the game by being able to prevent champions from using their damage in unintended ways. Dive fighters could be given the damage to still remain effective at engaging squishy targets in the backline of a team without remaining capable of also overpowering the competitions fighters and tanks as well.

Mages, assassins and ADC's could be given items aimed at increasing both their flat and percentage penetration without ever having to worry about other champion classes abusing the stats.

And fighters damage could be lowered significantly which would result in fighters actually dealing medium damage while remaining durable which would prevent them from essentially being assassins with more versatility and durability.

Might be a totally horrible idea but I thought I'd toss it out there and see what people thought.

EDIT- Though I did suggest what I typed above, this sort of mechanic could come in any form as long as it provides a dynamic interaction with existing stats linked to damage.

Null damage could also fix issues with current balancing methods. If you nerf a champions damage then naturally the overall performance of the champion drops, against everything. But if you were to change a champions mechanics like this, then suddenly you can balance the champion around other types of champions rather than having to either make the champion simply weaker or stronger.

If this change were to make it impossible for other champions to kill squishier targets then simply give them more null damage. It increases kill potential on squishy targets while remaining the same against targets with higher defense stats.

Squishy champions still under the thumb of everything else and their grandmother? starting handing out increases to base armor or MR.

Null damage still not affecting a bruisers champion ability to dominate tanks and squishies? Give it even further reduced effectiveness against its corrisponding resistance type along with the non interaction with pen. If the champions current armor rating gave 30% reduced damage against normal physical damage then it could have 45% reduced null physical.

These are all just examples of how you could shape something like that.