It would be very helpful if it were possible to open a playlist in the Now Playing right sidebar.
As worded, I am against this suggestion. My $0.02 follows...
It might seem like a convenient way to do it because the panel is already there but put simply the purpose of the Now Playing list is to list and manage the tracks that are now playing. Making its function dependent on whether the user is in "playing music mode" or "editing a playlist mode" can only lead to confusion (and bugs).
I can think of a few better ways to achieve the OP's requests.
1. Use the Now Playing AssistantAs this window already appears separate from the main track view, serves as a drop target, and allows items to be reordered by dragging it would a good candidate to also act as a playlist editor. It would just need some means of listing all tracks from a given playlist and also the ability to remain on top of the main MusicBee window.
2. Open playlists in a separate windowSimilar to #1, but just provide the ability to open a playlist in a dedicated pop-up window where it can be edited separate from the main MusicBee window. e.g.
Right-click playlist node > Open in New Window or
Right-click tab header > Open in New Window (which could be useful for non-playlist tabs too).
The previous two suggestions might work best for people used to working with a dedicated "Playlist Editor" feature in other programs.
3. Use the Now Playing listIt's already possible to open a playlist in the Now Playing List. Just play it.
For making
new playlists the Now Playing List does make an excellent playlist "scratchpad". It already serves the OP's request for a list separate from the library view where tracks can be added and reordered. Then it's just a matter of
right-click > List > Save as Library Playlist.
The problem is editing an existing playlist. You can load an existing playlist and add and reorder tracks, but there's currently no easy way to then propagate those changes back to the original playlist.
The suggestion here is create a new context menu item:
Replace Library Playlist. This would be a top-level playlist menu the same as
Send To > Playlist... works now. When a playlist item is selected from this new menu, it simply has all of its current tracks replaced with the tracklist currently shown in the Now Playing List. This should give no consideration to handling duplicates or appending to existing tracks; it's just a complete replacement. For clarity, the existing menu item should be renamed
Save as New Library Playlist.
#3 might appear to contradict what I said earlier about not overloading the function of the Now Playing List, but the difference here is that the Now Playing List still behaves exactly as it does now and doesn't need to know if it's in "playing music mode" or "playlist editing mode". It's only after the tracklist is fully assembled to the user's liking that they say "Thanks, MusicBee. By the way, I now want you to put all of these tracks into this playlist...".
The only downside I can think of with #3 is that it wouldn't be possible to play music
not in the playlist you were editing while doing so. It also has a whiff of "hidden feature-ness" about it, but no more so than the other "List" context menu items. On the other hand, I imagine it would be much easier to implement than any solution requiring a dedicated playlist editor.