Reworking Ashe's Passive in a Meaningful Way

As recent posts have underlined, Ashe's current "Focus" passive is wholly uninteresting and undertuned. Don't attack for a while, get a free crit... boring. Non- interactive. In my opinion, there are two directions they could take Ashe's passive - damage or utility. If they wanted to take it into a more damage oriented direction, as was suggested in a recent post, she could have a passive similar (or identical, even) to Talon's passive. You could also turn it into some kind of stacking mechanic, where she applied stacks with increasing damage to enemy champs with her Autos and Volleys.

Personally, I think a ramping move speed passive would be cooler, to reinforce (and modernize) her identity as a "kiting marksman." I was thinking something along the lines of:

Archer's Agility: When Ashe hits an enemy champion with an auto attack or Volley, she gains a stack (up to a maximum of 3 stacks) of Archer's Agility (lasts 3 seconds) that grants 2.5%/ 5%/ 7.5% bonus move speed. AA stacks decay one at a time, until refreshed by new Auto Attacks/ Volleys.