Similar to the upcoming Soraka visual remake and the past Kayle/Gangplank overhauls, Ashe is slated to receive an updatin'. This information was brought forth by Morello in a post where the community has demand that the Amethyst Ashe skin contain purple particle effects instead of the normal, blue spell effects. While this change is on the menu, I'm much happier now that I know soon I will not have to stare at Ashe's ugly and blocky feet.
So, I talked more to the skins team about this - something pretty exciting to add.official forums.
As you've seen with Gangplank and Kayle, we've been trying to redo some of our old, outdated-looking models. Ashe is next up on this treatment, and with that should come the Purple arrows.
The reason to wait for this is that particles, model and animations are all really closely tied together - and when we change the model, a lot of the "how things work" has to be redone anyhow. It's really simple to add it in there, and you get a sweet new Ashe base too!