I agree that MusicBee is in better shape than ever. I wasn't super invested in the GUI changes at first, but after using 3.0 for a while I have been won over.
The one major thing that I can think of for improvement at this point would be better artwork management. This would include:
*ability to see all artwork attached to a file in the main panel, or at the very least the number of images (for instance, a small number overlay that could be displayed on the artwork shown)
*easier access to and use of artwork sizes, both dimensions and storage space
*some better tool for editing multiple artworks than the current tag editor panel
*ability to know and control whether you're overwriting existing artwork when you save new ones outside of the tag editor
*ability to quickly see if all tracks from the same album have the same artwork (not just similar images, but actually the same)