I uploaded an animated GIF file by accident and noticed it played on the Upload screen.. very exciting! It doesn't work once the track is upload though.
If it was possible to allow us to use GIFs as art, people may really get into it. It would bring some life to the static soundcloud page without breaking the layout (like youtube's thumbnail previews) it sounds like a small thing but i think people would go crazy for it.
Perhaps a partnership with GIPHY? Or just letting user's upload .gif. Just a thought!