Ashe is supposed to be the CC Marksmen, right?
And the idea was that her crits are supposed to make that CC more powerful rather than amping damage, yes?
So how about this.
Ashe's critical strikes Root the target champion for (duration). If Ashe has Rooted a given enemy champion in the last (cooldown) her critical strikes instead amplify her passive slow against them by (percentage).
There you go.
Increasing her power via her CC, which is supposed to be the reason why you pick her, right?
passive on during her active Q and she roots and only she can proc it.