Brainstorming on Critical Strike and Life Steal
Zeal
+15% Attack Speed +20 Precision
Unique Passive: +5% Movement Speed
Precision: This value is added to the AD ratio of your champion's basic attacks, up to a maximum of +100.
Infinity Edge: Increases your Precision and Precision cap by 50%. Essence Reaver: Grants a flat 20% Cooldown Reduction, and restores Mana on-hit equal to 10% of your Precision. (Why does an ADC item grant more potential CDR than a mage item?) Statikk Shiv: Do something with its numbers, iunno.
Abilities that can critically strike (basically Parrrley) simply have a note saying that Precision is added to their AD ratios as well. The ones that critically strike for modified damage (Yi, Garen, Xin, Shaco) would need further tweaking. (Remember, Ezreal's Q cannot crit, and so doesn't count.)
Also, Yasuo and Jhin, but that's a lot of numbers to futz with.
For Ashe, remove the bonus Frost damage from her passive (since that's basically Precision), and make the slow scale with the Precision she builds, to a reasonable cap.
For Tryndamere, rework him already. In the meantime, though, I guess you can just change his crit chance to Precision directly. At least he won't RNG cheese you with crits.
I have no clue offhand how to modify Randuin's for this.
Also, since Precision's not as egregious and RNG a stat as critical strike chance, you could bring Zeal back as a Trinity Force component. 120% AD auto-attacks is easier to manage than "potential 200% AD auto-attacks on top of Spellblade procs". (If you do this, then you can make Nashor's Tooth build out of Recurve Bow and delete Stinger from the game because it's kinda pointless.)
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Life Steal: Cut all the Life Steal on items in half, and have it scale up based on your champion's Missing Health, with reduced effectiveness versus minions. (Also change the wording on Nasus' passive so it just heals him for a percentage of his damage but isn't called Life Steal.)
Basically make it so that all Life Steal works the way Warlord's Bloodlust does.