ADC itemization will always be broken for this reason.
They're all made to multiply each-others effects.
What ADC itemization, and itemization in general, should do is improve a champion's kit over-all.
Master Yi, Irelia, Jax, etc are all fantastic example of this. Items are bought to scale their kit, not just each other so their auto-attacks become powerful by default. Most mages fit this bill pretty well, but there needs to be less
Where they're just stat-sticks and more
, where they provide actual utilities.
The thing that makes Yi, Irelia, and Jax great examples is how they're focused on auto-attacks, but its an ability related to them that's being scaled in effectiveness rather than the items themselves scaling each other. They multiply the effect of the kit, not the other items. They are what real auto-attack carries should be like.
This is why ADC itemization and
+
are so fundamentally bad, and why Fervor of Battle is a failure, IMO.
, while being better stat-wise than what they counter ATM, are fantastic concepts because they improve your ability to reliably apply your kit, rather than just improve AD damage. The issue isn't these items, its how kits interact with other items, making this game's static flow of Assassin>ADC>Tank>Assassin far too binary when either assassins can't burst or people can't burst ADCs. This binary flow due to League's overarching metagame as a result of itemization and base balance concept (AD vs AP critical mass stat interactions) is what makes these items too powerful.
If AP and AD itemization was more about improving aspects of one's kit or covering flaws, as opposed to "hurr more damage", you would find many more champions are opened up to viability and ADCs would stop being 'Let's see who's the safest/most powerful auto-attacker this patch.'
It also makes it so 'assassins' are another form of engage, damage, as opposed to the quintessential 'assassin' insta-gibbing someone. You would follow-up on damage, rather than CC in this case. They could be balanced around this and still be a fulfilling experience for everyone while also retaining the '1-shot' ability while extremely fed, rather than trying to just instant-kill someone when they out-stat them or are out of position. Item actives would be required for 1v1-killing more, rather than a kit just having the ability to kill someone 1v1. This means real costs for all-ins, rather than just 10 second CDs. Itemization would be the gating mechanism, as opposed to just hitting level 3, 6, 9, 11, or 13 depending on the champion.
An example of how to make AP damage items that aren't just stat sticks are items like Refresher Orb, Orchid Malevolence, Dagon, and Aghanim's Scepter-like items from DotA 2. Yes, they would have to be made to fit League without being broken, but options for damage without just making someone out-stat someone else are there. Azir is a good example of what could happen if there were more itemization that affected the kit, rather than just numbers. Yes, Azir is poorly designed with how he functions, but it would feel really good for the player and be less frustrating for the opponent on any other champion when they all can improve their kit, rather than just the damage it provides.
An example of how to make ADs more varied is by varying their kits and having items improve those, rather than solely their auto-attacks. If auto-attacks are the main factor of ADCs, you will always run into the meta issues of who applies damage most effectively, and only those champions will be used, rather than ADCs being used for what they offer overall.
A world where ADCs are balanced is a world where they and bruisers can build the same items and be considered balanced and have trade-offs for their range, as opposed to the mess we have now because of how well 2 or 3 ADC items scale each other.