Rune idea; Legend, Accuracy.

As you kill things, you gain a small amount of critical chance. (starting at 5% and slowly growing to 20)

thought this up while que'ing to try out buffed kha'zix. maybe someone else thought of it too, maybe not, idk.

the basic idea is that not everyone needs IE, ER, and their choice of other crit items. this would help round out their crit chance, while making them give up their legend; alacrity or Bloodthirst.

and yes, it may add a liiitle early power to yasou, but it wont be suuper broken as he'd still build IE/Phantom anyway.