I'm not sure if this is possible using XenForo engine... But having a possibility of an animated gif as avatar, both full-size and miniature, would be really cool and many users would probably appreciate that. Of course while saving the animated part, because AFAIK .gifs can be uploaded, but they won't animate at all. I've added a poll, closing after 14 days. If the time's too short, feel free to change it.