I have a question though. Can you confirm "Music" "Soundtracks" etc are tabs (and not a predefined set of all the choices that might have been found in the left navigator)? And clicking +, how to you see the user selecting what they want to be displayed in the tab? (perhaps the main panel fills with all the choices in a grid such as: Music, AudioBooks, Podcasts, Playlists etc for the user to choose)
I like having the left panel in first screenshot because i can imagine when Playlists is selected all the playlists display there or if Computer is selected all the computer folders display without having a special panel opening like in iTunes12 does for playlists
Maybe "Music", "Soundtracks", etc should be filtered Music Library tabs.

Not sure about this one, I think it still needs more thinking.
Regarding the fourth "artist view" idea, one way would be to separate the current group by settings in artwork view to album and all sorts of artists. Then "artwork view" will be always grouped by square album picture, while "artist view" could be grouped by (album) artist and (sort) composer with round artist picture. If you click on an artist picture, then it would open up the 2nd image of the proposal.
My idea was something like this:

Should be easy, just a inserting a header whenever the user is viewing a single Artist. The content panel doesn't even need changes
Also, a new view (albums + sidebar) could work for all the library, not just artists.
Note: Maybe the Artist Header should be smaller?
DEVICES would auto-open as a tab but i need to think some more about that one.
That's probably one of the hardest parts to resolve
- for the right panel I see the first screenshot as a new view "Playing Album v". You could also choose "Playing Tracks v" which is similar to now where it would show the now playing list and have one panel beneath that has a button in the same style as "Playing Album v" in the screenshot with the choices: "Lyrics v"/ "Track Details v"/ "Artwork v". In addition to "Playing Album v" and "Playing Tracks v", there would also be "Selected Tracks v" which would show the selected tracks where the now playing list shows and the artwork/ lyrics/ track details for the selected tracks
@_@
I have to read all this paragraph again, I'm still confused

BTW, the right panel on the images is just showing the current "Album and Tracks" view.
And a mockup of how the "Albums v" button could work:

Each panel should have one "Title Button", with their respective options.
So I hope that development of MusicBee will not go in that direction where it is very important that it looks contemporary enough, and trying to please both PC and tablet users somewhere in the middle.
On my desktop I mainly want all the information and options of an application available as fast and easy as possible.
On my tablet or phone I wouldn't mind when many advanced options were hidden or not even be available at all.
So what I am probably trying to say here; please don't make too many changes for using MusicBee on a desktop PC, since it is close to perfection already.
Any effort in this area might better be spend on developments for tablet (and possibly smartphone).
(and probably would also bring you a shitload of well-deserved £ when done right)
Most of the changes are not drastic, I think.
When I talk about a more "touch oriented interface", right now I'm just talking about this:

Left: Current Version / Right: Proposed.
Just playing with the margins of some elements could make a big difference for some touch users. Other Windows apps have already done this, like Google Chrome.
Yes, it's still a Win32 app, so as you've said, it doesn't make sense to go "full touch" and responsive design, that's what the new Universal apps are for. But as a Win32 app I think MusicBee could benefit from being a bit more touch-friendly in Windows, specially now that Universal apps on "desktop mode" behave almost like a Win32 app (can be windowed, opens normal dialog windows, etc).