It would be really helpful to be able to create custom folder types for the Asset Pane. Specifically, the ability to create a custom folder with an allowed types attribute. The final goal being to enforce Asset Pane organization. The Asset Pane can easily become very disorganized if content editors are not diligent. If developers could enforce some order it would be great.
There does seem to be a way to create a top level custom folder, but no subfolders. And these custom folders were not very flexable.