E.g. dragging and dropping a file or folder with sub-folders with music files in them.
Area to drop - e.g. playback panel (panel with play/stop buttons, progress gauge etc.).
Windows Explorer right-click menu would be nice as well (for folders as well so that you could select multiple folders and send them to MusicBee) - play in MusicBee (create and start new PL), some other options : Play Next (Insert song(s) after the currently playing in the PL), Play Last (adding to the end of the existing PL)