I suspect it would be surprisingly easy to accidentally push Ashe into OP territory. Yes, Ashe has the lowest potential damage output of any AD carry and technically has no repositioning skill (I say "technically" because a 4 second cooldown aoe slow + a single target permaslow on autoattack can be pretty decent for repositioning in a lot of situations), but in exchange, she has a huge amount of utility. Permaslow autoattacks, aoe slow on a 4 second cooldown, built-in clairvoyance, and her very own teamfight initiation ult. Giving Ashe a strong damage passive could very easily make her OP.
I think an interesting alternative to Ashe's current passive might be:
Ashe gains x% crit chance for every autoattack since her last critical strike.
It would make her passive useful for something other than her first attack when getting to lane or her first attack in a teamfight. The exact percent would have to be toned down from where it is now (18% lategame would be too strong), but it certainly seems like an overall better passive for an AD carry than Ashe's current one.