If armor is too much of a problem in earlymidgame but falls off to be completely irrelevant lategame

Change the formula, or buff the passives (Frozen Heart AS slow, Randuins AS slow, Randuins crit reduction) when you nerf the base stats. Instead of straight nerfing the items. Yes they're good when you get those in a matchup, but think about it. If you're a tank, you'll need to get MR as well (which will be a fully useless stat against AD but you have to build it) so overall if you wanna be tanky you have to split your build in several directions that do not stack with each other (adaptive armor mastery is way too weak to compensate that problem).

Also, armor falls off very hard compared to physical damage, which has great synergies with AS/Crit/Arpen/lifesteal/ cdr for AD casters

1. About the formula, it is currently Damage received in % : 100 / (1 + resist/100) resist being armor or magic resist

You could change it to something like 100/ (1+ (resist/100) ^ anotherstat)

anotherstat will be a number extremely close to 1 (I wouldn't get it higher than 1.2/1.25 or it's gonna be too strong), but a bit superior which is either a fixed value, or could be slightly amplified by some items. This will allow an exponential growth of armor. The change is unnoticeable with low amounts of armor and can be seen when you have really built very tanky (and resists kind of tanky, not meat ball of health -which would actually benefit more top tanks than jungle tanks, by a small margin-)

EXAMPLE :

anotherstat = 1.15

You have 300 armor old armor : damage received 25% new armor : damage received 22% new effective armor : 354

AND (good point) it doesn't serve the scalings on thornmail for instance for potentially ridiculous returning damage (though I highly doubt it could ever be a problem, especially since thornmail is a bit weak and situational right now)

Considering we can only have 6 items in inventory, the exponential growth is not a problem, since we're limited to a number of armor still too low for the scaling to be off the charts (and things such as 10 hours of farming souls Thresh are irrelevant in a usual game -they'd be op anyway without that change-)

DRAWBACKS OF THIS METHOD : nothing done against true damage, and this would indirectly buff champions that are already kinda problematic damage-wise( )

2. If we wanna keep the formula how it is right now, and we think armor is too strong early-mid and not quite relevant enough late, give strength to the passives given by items. For instance, Frozen Heart has been straight nerfed on both stats and cost. Come on (!) Buffing its passive from 15 to 20% would make it scale a bit better in lategame while still being nerfed on armor value and not quite as obnoxious as pre-nerf. Randuin's omen has averagely good passives for DPS reducing that scale into lategame (as reduction, crit reduction). Buffing one of them would increase the scaling of this item but its base stats remain nerfed and therefore not that great as a single buy.

DRAWBACKS OF THIS METHOD : Indirectly buffs AD casters/assassins based on spells. Possible countermeasure : create or adapt an item to include damage reduction from spells? physical damage spells?

End words : DO NOT think that this will make ADCs irrelevant in lategame or that they won't be carries anymore. This will just help a bit tanky champions not to get utterly melted as if their resists and health didn't quite exist at all while limiting the fact that tanks are usually full midgame-focused and fall off so hard in lategame. They'll still get kinda melted , but a tiny bit less.