Make armor pen/shred/lethality exclusive to champs that buy them.
At the moment the "best" champs in the game are the ones that can stack all 3 lethality items, a LW item and find a way to put black cleaver in their build. That way they deal true damage to anyone with less than 100 armor, and near enough to true damage to anyone who isnt an armor stacking tank.
Now I'm fine with the principle of lethality (which is just renamed flat armor pen). You stack enough of it, you deal true damage to anyone who doesn't buy any armor. You excel at killing squishy champs. League saw the same thing during league of black cleavers when the armor pen wasnt unique and you could stack 5. if you want to stack a certain stat to excel at killing a certain type of champ (those with low armor/health) then there is nothing wrong with that. The same goes for it you stack crit/AS and % bonus armor pen because you want to take down the other teams frontline.
The thing that I have a problem with is when it becomes viable to pick up this single stat and have it good against EVERYTHING, making building against it useless. So when a champ stacks lethality, and LW and black cleaver so that they delete squishy champs yet still deal amazing damage to people with 450+ armor, that is when the stat and the items associated with it have a problem.
What could be done?? Riot could make it so that Lethality is applied before % armor pen, as it was before season 2. And they could make it so that the armor shred of BC and the % armor pen of LW dont stack.
If they really wanted to take a bold step they would just make the 3 things mutually exclusive so that if you specialize to kill someone, you dont double dip to get bonus's on something else. So if you built lethality than you dont gain anything from % armor pen, or BC's armor shred. And vise versa. So if you're building for 600 Varus Qs on the enemy backline, you dont deal as much to the enemy frontline. Or if you build Cait to shred the enemy frontline, you dont get to benefit from lethality if you Q/ult hits an enemy backline champ. Or if you name is Jayce, you don't get to triple dip and ignore armor altogether.
Because on their own there is nothing wrong with these stats, its when there is nothing barring you from stacking them that problems arise. (FYI
is 285 bonus AD, with 50 lethality, and 45% bonus armor pen. With black cleaver, and lord doms passive. Against a 500 armor tank (probably with higher health) just from these items alone they are going to bring it 142.5 armor for damage caclulation.)
) then they end up with 65.5 armor. 300-90=(210*0.55) - 50 = 65.5
