The fundamental problems with the ADC class from an ADC main

Problem 1: Critical Strike being accessible to every champion: it mostly eliminates the distinction between "late game and early game champs" into " strong late game champs who are strong early game and incredibly strong late game champs who are weak early game". This also hurts diversity among ADCs because guess what gets picked more (unless the incredibly strong late game champs get tuned to have strong early game, thanks love the free elo <3)

Solution: Remove Critical Strike from items (crit on Zeal items would be replaced with AD and BoRK would become a Zeal item), compensate hypercarry-style ADCs in their kits (this alone would require preseason level balancing work), push early game bullies towards AD/Arpen builds.

Examples: Jinx gets an extra 2 AD of AD growth, and her minigun gets 2/4/6/8% stacking arpen per stack. Vayne gets an automatic Critical Strike on Tumble Kog'Maw can actually stay as is given he isn't truly crit reliant Twitch may not even need compensation buffs given the nature of his ultimate Tristana prob needs a rework Xayah would probably need some changes to numbers to make her AD ratios stronger

Solution 2: (less drastic) - Severely reduce ASPD growth of champions like Lucian, Jhin, Miss Fortune (full disclosure, my main), slightly increase AD growth. Similar to the treatment Caitlyn got (as an aside - i think Caitlyn should be building Lethality).

This forces them into purer AD builds and makes speccing into crit scaling less effective late game, as they simply can't DPS at the rate needed to cut through tanks and ends up falling off late game. MF needs more work though because her build with Cleaver both does too much damage and has backline access (personally I favor partially reverting her rework and bringing back an updated version of Impure Shots).

Problem 2: Assuming we're keeping Critical Strike, we have to address how Riot's push to have shorter, more snowbally games absolutely fucks game balance. ADCs are balanced around weak/weakish early game, strong late game. Strong late game is somewhere at 3 items + upgraded boots. That's generally been around 11k gold, more than the other classes. Assuming the ADC isn't getting fed and farming at an average rate, that's somewhere around 30-35 minutes The problem is that if you're balancing games to be decided (not ended, decided because even if the nexus finally blows up or the team surrenders at minute 33, the outcome probably hasn't been in doubt for the last 5 minutes at least, possibly the last 10) between 20-25 minutes on average. Couple that with the more snowbally games, the outcome is being decided before the ADC truly comes online and can influence the game equally with the rest of the team. Thus: ADC in 2K17 lul.

So Riot decides to solve this problem (after ADCs try to paper over this problem using Lethality) by drastically cutting the cost of ADC items. This of course is basically giving ADCs 2 free kills or 40 free CS in terms of gold value, and this is on a SCALING role. While ADCs get to play the game now in this new shortened metagame, having ADCs spike so early kind of just turns the game from ADC in 2k17 lul to everyone else in 2k17 lul. This is compounded, of course, by and other enchanter items designed to make the ADC even stronger.

Solution: Again, needs major work to fix this. Needs a lot of map updates to be reverted - Rift Herald probably has to go, first brick gold probably needs to go, or at least rebalance the def on the towers (Riot will have to accept that lane swaps will be a strong strategy), Baron probably needs to go back to its prerework state instead of becoming a pushing tool, and Dragon needs to go back to one of its previous states (either the stacking Dragon or the gold giving Dragon). Also bring back the old warding system - sure it also helped snowballing when you are ALREADY snowballing (because you had more gold to spare for wards) but it also impeded snowballing because you had vision to not die as much. Riot will also have to accept that a team will pick Ziggs or Anivia and play for 40-45 mins and win that way, which they didn't like because people want to fight and brawl not play strategically (or watch fighting and brawling not strategic play).

Of course, removing Critical Strike (if we choose to go that route) helps here, and then you can rebalance ADCs accordingly, but you still leave a balance concern where those new lategame ADCs you reworked never get to live their power fantasy because Lucian or MF shat on them in lane and the game is over at minute 20.

Problem 3: ADC being mandatory is a product of the design of the map - you have 4 gold income streams and 5 players. Therefore some player is going to need to get significantly less gold than the rest of the team. So the map itself locks you to "4+1" so you're ALWAYS going to have a support. But you might ask, why not play another role with the support? First off, every other class prefers levels to gold for the early parts of the game, so running bruiser + support (and double bruiser bot is even worse unless you snowball), and while some mages can work down there (not just Ziggs but also Cass and maybe Azir), and a few melees designed to work down there (Morde, Yasuo to an extent who was designed to be a botlane melee carry) ranged ADCs are really the only class that is optimal. ADC's mandatory nature is less of them being "overpowered" and more being the only puzzle piece that can fit a particular slot.

Solution: Again, requires major, deep changes to the map. Ideally, making ADCs less mandatory in bot would allow them to be more consistently viable (not meme counterpicks) in other lanes. Maybe making bot lane shorter could help, or making more gold/not level champs from other classes could help (but they'd have to be explicitly designed for this role). Other roles can't go botlane not just because other roles have issues laning against ADC + Support but because other roles will fall off against ADC + Support as the ADC buys items and the other role lacks the levels they REALLY need.

Problem 4:

Lucian. He gets his own category because he's a fundamentally broken champion in the context of class balance and his kit has to be gutted every few months because whenever he's remotely strong he instantly takes over the meta. He's smooth and fun to play but he's also broken. He more or less has the strengths of a mage, with few of the weaknesses, also he effectively has Riven's passive with higher base stats. Even when his WR isn't high his playrate is skyhigh.

Solution: Make him an AP scaling mage/assassin hybrid. Second shot does 50% of AP as magic damage, Q now scales entirely with AP, remove AD ratio from R. Maybe increase CD return on E. Simplest solution here.