I can't believe I'm saying this but armor does absolutely nothing
Before I begin, I have to say I hated tank meta. It was one of the worse things. The fact that you can do damage while being insane hard to kill, to the point where you could tank nexus towers and 5 champs and still be taunting and dancing while they hit you but also still dealing a good amount of damage. That was stupid to play against especially when someone could get that strong even though they were feeding all game since defensive items are cheaper than offensive.
But as of now, armor literally does nothing. You can have 100 armor and have physical damage reduced by 50 percent, doesn't even matter. Someone could build full damage, literally no last whisper or black cleaver and still shred you. It's kind of ridiculous and there's no point in building armor imo anymore unless maybe you are playing taric and rammus. I could have 300 armor, 300 magic resist, 3k or more hp and still melt in a matter of 5 seconds during a teamfight or if i get caught out.
I do however think that being tanky is a good thing that should still exist, but to balance this, any items that give health, armor, magic resist should reduce the amount of damage you can deal. This is the only way to make tanks fair because if you make them hard to kill, they will still have a lot of damage in their base stats and abilities vs a squishy adc, mage, assassin. If an item makes you harder to kill but makes you deal less damage, it would be fair since you sacrifice damage for incredible survivability. During the tank meta, adcs were having trouble doing damage to tanks relative to how much tanks did to adcs. The keyword is relativity. Even if an adc was hitting for 500 damage, the tank had more health while if the tank hit the adc for let's say 300 damage, the tank is winning since the adc has a lot less health. So even if the adc was technically dealing more damage, the tank is actually dealing more damage percentage wise because the adc has a lower health pool. The reason for this is because base health and resistances are vulnerable to base damage and abilities, while high resist and health was very resistant to high damage. That's why during the tank meta, everyone was building tank because it made those building offense to do very little damage to them, while they were able to do damage to people building offense because they were squishy.
Good example is how defensive build will beat offensive build in a 1v1 and is cheaper back in the older seasons. I played katarina vs katarina mid once, and I was building offensive items like death cap and void staff, but I got my ass kicked because the other kat rushed banshees. Keep in mind this was the old banshee that didn't give ap but rather just magic resist, health, and the shield. I understand why I lost the 1v1 but it's stupid that the other kat can beat me buying a defensive item that gives no offensive stats vs me going full offense. I think in a 1v1, the person going offensive should win against a person going defensive. The only reason you go defensive is if you are losing lane, you want to survive as long as possible to provide utility and crowd control, or you want to tank for your team. It shouldn't be that you build defensive to win 1v1s. That's just kind of stupid. The only reason it works that way is because base health and resistances lose to base damage and abilties so that's why I was losing to a kat who rushed old banshees while I went void staff or rabadons first. But in my opinion, it should not be that way.
I loved being tanky and hard to kill in older seasons, but definitely felt it was not fair that I could hurt squishy people without building any offensive items. So a good way to balance this is just to make tanks very tanky again, or at least a little more tanky than they are now, at the cost of them dealing less damage when they build those tanky items because those tanky items will reduce your damage by either a flat amount or percentage. Maybe make the defensive items lower your ad/ap. Or make it just have you deal 10 percent less damage etc. That would make it fair but not make you useless as a tank because you will still have disrupting abilities like stuns slows knock ups. A tank shouldn't be able to 1v1 someone going offensive in an all out fight, but a tank should be able to be hard to kill and tank towers and objectives.