can we make lethality and % penetration just not stack?
both stats individually arent problematic.
you either stack lethality to deal almost true damage to squishy targets and blow them up, or you get % penetration against extremely armor heavy teams.
its when youre allowed to combine both stats to synergize with each other, where things get obscene.
buff the % armor penetration mastery back slightly and move it to the cunning tree, to give a certain baseline of armor pen to lethality stackers, and then be more generous with same name passives on armor pen items.
just make the big ones not stack with each other on the same champion

i realize this would take a lot of rebalancing of armor pen- and armor items, but in the long run it would be a much healthier approach.