Maybe instead of crit...
ADC items could work like Diana passive. Zeal items could give a window of extra attack speed. IE, Reaver could give extra damage for 3 seconds or something. Last whisper for extra pen. Most adc's cast spells throughout combat, so this could get rid of the rng factor. Assassins like to use all spells in a burst, so not great items for them. If you need more scaling, have them amplify their effects with multiple purchases. Melee carries still need defensive stats, so shouldn't take over late game. I'm just thinking this could be less rng and perhaps an overall flatter power curve for adc.
AD and increase its price, focus
on waveclear and sieging respectively instead of turning them into mixed damage hyper threats and remove crit procs from shiv, lower
AD.