I think I've come up with a good Ashe Passive
Focus
While out of combat, Ashe gains stacks of Focus. When Ashe has 100 stacks of Focus her next basic attack will critically strike.
My Thoughts
As an AD Carry, you should have the frame of mind that whenever you are at an appropriate range to Auto-Attack an enemy safely, you should be doing so. However, Ashe's current passive encourages the player to do the exact opposite - it encourages the player to stay out of combat, which is counter-intuitive.
My Proposed Changes
For each second in combat, Ashe gains focus. For each stack of Focus, Ashe gains a % Critical Strike Chance (based on level) up to a maximum of 5 stacks. (Bonus Critical Strike Chance at Level 16 is 25%). % Critical Strike per second increases by 1.25 per rank at levels 1/6/11/16. [(1.25/2.5/3.75/5% per second)(6.25/12.5/18.75/25% @5 seconds)]
What this Changes
In the early game, you should be focused on maintaining a balance between last-hitting minions, and poking with Volley and Auto-Attacks. What this means is, that around every 15 Auto-Attacks -meaning you crit as often as you do currently-, you should get a Critical Strike which you can follow up with a Volley and potentially hard engage with your ultimate.
Later in the game, this passive becomes more of a factor where instead of Critically Striking on 5/10 Auto-Attacks, you would be doing so for 6-8/10.
and
you'll have up to 70% crit chance, and with
you'll have up to 80% crit chance at level 16 with max stacks.