The current playlist save format (m3u) does not allow to save all information of the paylist tracks, especially the tree structure.
XPSF seems a good format, quite simple though powerful through the use of tags.
In my dreams, the saved playlist format of VLC would remember ALL properties set by user for each track, ie all menu options, volume, image propertiers, stream output, etc, and of course the tracks order and the tree structure. And it would be the default autosave format.
Note: xsl stylesheets exist to convert xspf to m3u/html/smil.