Continue playback when closed
I think it might be a QOL improvement if GNOME Music had the ability to continue playback without needing to have the window. Most people put on music in the background while they work, this will allow them to focus on work without having to manage the music player window in their workspace.
I am not sure if this could be against the GNOME HIG though, since I just read a similar discussion about Fragments. Nonetheless, this is a common behavior supported by most music players across OSes, and is also supported by Lollypop.