To solve the problem of tanks and antitanks we need exponential scalings

To make a tank item REALLY only efficient on true tanks and not on pesky bruisers, since stacking resistances does have diminishing returns (1 point of resistance always grants +1% of your visible HP, which the more resistance you have the less it represents over your actual hp) you need to give it exponential/square scaling. Obviously, the word exponential musn't be used since it will scare people off because maths, so let'ss just say double scaling. An item granting something equal to x% resource X times y% resource Y (which can obviously be the same as X). And if you want some champion to really punish tanks, what has been done on Cait is not going to help. Ignoring 50% of bonus armor is just going to hurt the armor seal stacking ADC during laning phase almost as much as the lategame tank, making Cait even more of a noobstomper (playing against someone who has much higher range is already not an obvious thing to do, if you add to this that she gets to do a crittable 150% of her AD that ignores the little armor you have...). You need double scaling once more.