I wish I could drag
a track by clicking the artwork in the Now Playing Panel, or the song title in the Player Controls Panel, and then be able to drop
a playlist in the Navigator Panel or into
the playlist shown in the Main Panel.
The areas marked with red is what I suggest is to be "drag-able".:
I can see two situations where this would come in handy:
1. You are currently listening to a track and in the Main Panel you are viewing a playlist which does not have the track but you think it should. You drag the track into the playlist.
2. You want to place the currently playing track into several playlists while browsing a website, another playlist or something else you wish to keep in the Main Panel. You could then drag the track onto those playlists while keeping the Main Panel intact.
As of now you need to "locate playing track at source" by double clcking the track title in the Player Controls Panel, and then drag the track to where you want. To get back to where you were in the main panel you would either have it stored in another tab or get there by the Back button.
For example, the music player Spotify uses this and its really comfortable