The common theme when it comes to balance changes is to look at champions who over-perform or under-perform across the various leagues and identify if it's a scaling issue related to the specific champion, or if it's an item or keystone issue that has too much synergy to be reasonably played around. Which seems logical from a top down perspective to be the right way to go around balance changes.
But with that said, you've admitted to item and keystone changes having wide reaching consequences across a myriad of champions that makes balance changes impossible in a vacuum, as adding X to this item would make it a must buy on Y champion, making them too strong, or N change making an entire group of items useless on M champion, making said champion useless.
We've also reached a situation where ranged ADs, by and large, whether statistically accurate or not, _feel _ completely useless in game. The insane amount of damage creep, mobility, and the itemization changes to push their powerspike later into the game have left AD mains in a rough spot where either the game is over before they "come on" or they are abused all game to a point where they never come on, because the three item powerspike is the equivalent to the two item powerspike of most other, in favor, champions.
It's also placed us in a situation where ranged ADs in the bottom lane, lacking in its pre-three-item damage of past seasons, rely on mage or high damage supports to make up for the deficit, throwing enchanter and tank supports out of the meta, outside of a few high-cc champions that can abuse the lack of mobility in most AD's kits. This isn't the case everywhere, as many professional games still use tank supports, but their games, on average, tend to go long enough for their AD to become useful, and their coordination as a team lends itself well to their compositions, as well as the normal professional player being very, very good at dodging.
So with all the above stated, why not just create ranged only items?
Certainly, it would be extra work. But it would provide an easy avenue for tuning specific items that only affect a specific group of champions. Gone would be the necessity of making sure changing a certain item wouldn't make Master Yi an unstoppable god, and thematically, we would stop buying swords to make our bows and guns stronger. You could also finally do away with the confusing subtexts on items as % decreased for ranged champions, or unique melee only modifiers.
It would also give players more itemization options to play around the issues they face in their current game. Ranged ADs wouldn't have to make a decision between two items that have unique passives that don't affect them, or as well as they'd like, but still have to pay the gold for the mechanic. Take for instance Duskblade, a good item for ranged ADs building lethality, (certain Jhin, MF, and Lucian builds), pay a premium for a mechanic they can't even use. Frozen mallet, Guinsoos, Phage, DMP, Knight's Vow (a _support item_), and Wit's End, all have a ranged penalty. This is at least a realization and admission that these items affect different classes of champions differently, so why go through the extra steps and coding when the valuation for these items is created for the full item effect regardless of who purchases them?
Riot, you should take this opportunity to do away with the complication and balance issues surrounding these items and create more items like Runaan's Hurricane which effect only the class of champions you're trying to adjust. Not only would this make your lives easier in the long run as you would be able to fine tune the specific items affecting specific champions without being required to take into consideration the wide reaching consequences of other champions, it would give a struggling class of champions the opportunity to itemize the way they want and need depending on specific matchups, it would also be a thematic change, where not as important, Jhin wouldn't have to buy a sword to make his bullets do more damage.
Obviously this would be a large undertaking, but if the goal is to create a balanced game that is as fun to play as it is fun to watch, I think it's something you should at least consider.
For fun, I've included a poll. Given my thoughts, and any thoughts you readers may have, would it be a good idea, for the longevity and ease of balance moving forward, knowing the growing pains therein, to have a set of ranged only items?