put some diminishing returns on armor penetration.
Right now. here is how armor penetration works. it has no diminishing returns. wheather its percentage total or pecentage bonus of flat armor pen. however you get it it stacks additively. now its really stupid when a miss fortune gets a black cleaver and a last whisper and there goes 2/3s of my armor. no need for that. we have diminishing returns for stacking armor. this doesn't feel right.
for magic pen. there isn't a way to get a whole lot of it. and mages don't auto attack you