Suggestion to fix anti-tank problems

People are asking for the Last Whisper item to be nerfed because of how fast it takes down tanks.

But nerfing the item would only buff EVERY CHAMPION WITHOUT TANK BUILDS.

So why nerf the item instead of just making a new one with a new mechanic?

The Last Whisper does +35% armor pen.

So let's say you make a new item, something that negates part of that.

Say a character does a total of 60% armor pen, for some odd reason. But the new item you introduce is anti-armor-pen.

Say the new item does 15% anti armor pen. This means that Last Whisper now does only 25% armor pen BUT only to the person who owns that item.

In this case, Vayne has armor pen total of 60% with her Last Whisper, runes, and so forth.

Azir over here does not have the item in question, therefore she will take down his health without his armor because his low amount of armor is run over by the 60% armor pen that Vayne has. Which allows Vaynes Last Whisper to be worth it against tanks and non tanks.

However Nautilus in the other corner DOES have this item. For the sake of organization I'll call the item Mr.Clean

Mr.Clean has +150 health +15% anti armor pen.

Nautilus buys his Mr.Clean and takes on Vayne, with her 60% armor pen. However because of Mr.Clean her armor pen is reduced to 55% and allows Nautilus further survivability against a champ who would normally almost completely ignore this 85% AD damage reduction.

I'm not very good with League math, so someone needs to come up with some calculations of how much damage they would do with and without these items in play. But I think this might solve the issue everyone is complaining about.