Currently, each of these has an entirely different library with different favorites, playlists, etc, which makes it a little bit difficult to manage everything. I use iTunes for my local files, then Spotify has its own library of "liked" files, YouTube has its own playlists, SoundCloud has its own, and so on.

I would love for there to be some kind of simple app that lets me build a music library that incorporates all of these sources. I'm envisioning something kind of iTunes or Winamp-esque, except instead of the elements having to be files, they'd instead just be tokens that could point to a local file, or a remote HTTP URL, or a spotify: URI or a YouTube video or whatever. I would be able to put some kind of basic metadata on each one and organize things into artists, albums, and so on, as well as make playlists.

Does something exist that can do this? I feel like this could be a pretty simple web app, particularly if someone is willing to use e.g. FTP or Dropbox instead of a local file.