Wouldn't that mean that you'd have to make a new avatar for every new user once the current icons run out?
I'll consider this - hopefully it wouldn't be too difficult to develop or hire someone to do it. No extensions with this functionality exist out there that I can modify for barrens chat at the moment. I'll do some digging on github and see what there is available.
In the meantime, there's more than five thousand to choose from so it shouldn't be an issue - however I can see what you mean. I reckon some avatars will be more popular than others, and since you are restricted to choosing one from the gallery as opposed to uploading your own, they could loose their unique feeling.
There is an issue that came up to my mind - inactive users who had "booked" the avatar and never returned to the forum. Might be really sad.
To be honest, I agree. Well said.