Critical Strike System is the root of ADC's unbalance

Riot let this outdated system go on for too long. That's why heavy critical hit ADC's (Twitch, Tristana, etc) become so strong when IE and Zeal items get buffed; that's why they have always been the strongest champ niche in terms of damage (together with some of the "Skimishers"). They're only more impactful now days because of the recent support's shielding/healing over buff, making them too resistant and strong.

Maybe rework it in something ""similar"" to Ashe's passive, but instead of a slow on crit, something good but fair and balanced.