Given that a playlist generally necessitates a different UI, it would be non-trivial (though possible) to implement as a plugin. I'd be curious to understand, in your use case, how you would divide the responsibilities between player and plugin. Should the player know about playlists and define the UI for it, or should the player be completely agnostic and have the plugin centralize that logic (including UI)?
I my case, I was thinking, if the player load the plugin´s, (im using reops xml config to load plugin, eg. tracking plugin). So I have a 'standard player', which can have plugins added on..(jw player likeish way). In the case playlist the player would be call with an id to an entry in a feed, the playlistplugin would then in some cases get eg. the category of that mediaelement (i would suspect in the form of metadata) and load a feed and display a playlist, thumbnails title ect... when a item is selected it would change the players mediaelement (im not sure how to do this from the plugin!). So in the case the plugin would be centralized..
The reason for plugin is to make the player flexible, so the player could be called with playlist or other plugins.. Im excited to hear what is best practise regarding playlist or other way to inplement playlists..