The main point that needs to be done, assuming we're going for a complete do-over of the ADC class for balance purposes, is to shift a lot of their lategame power for early and mid-game, depending on individual champions so that they no longer require babysitting. That means base HP buffs, base damage buffs, abilities with stronger base damage, scaling nerfs to their kits, and nerfs to most of their methods for AD/Crit/Lethality stacking.
Assuming Riot did perform the kind of Crit rework i've suggested earler on, it'd make a near-perfect lever to control when and how ADCs attain their powerspikes.
Some, like Kog'Maw, Kindred and Tristana who are 'hypercarries' can keep most of their late-game awesomeness, but could still use a slightly better early game to be able to get there on their own.
Many others, like Graves, Vayne, Sivir, and Xayah should likely shine most in the midgame where most of the brawling occurs as they can use their innate defenses or mobility to survive fights.
Others, like Caitlyn, Miss Fortune and Lucian would do best in the early game, where they can bully the enemy laners and gain some amount of snowballing.
An example of how Kog'Maw could be made viable mid would be to grant him a Crit autoattack every time he lands his R, limiting his early-game power but giving him a power spike when most Midlaners become most dangerous. (keeping in mind his overall early game has been slightly buffed from current)
Jinx in her current state really only needs a way to sustain Fishbones' mana cost to be viable anywhere. Riot could either buff Manamune to start out stronger, with the same end result. Another option would be to create a mini-Essence Reaver (like Exec's Calling is a mini Mortal Reminder) that gives champions a bit of mana back on champion hit. Or they could lower Fishbones' mana cost too, that'd work.
