I've been using MusicBee for about 5 years now. I've never found a better media player, especially with this many options for customization. That said, there's one thing I would love to see expanded upon.
I'm pretty particular about album art quality and also pretty particular about maintaining consistency across my collection. I save the highest quality source images I can find and resize them in GIMP to 700x700 pixels, which I use because it's the standard size embedded in Bandcamp downloads and I buy from there a lot.
After resizing them I use the "Paste album cover" feature to set the image. However, I've noticed that once embedded they come out with more JPEG compression and artifacts than the original image I copied and pasted.
I assume this is done to keep the filesize relatively small, but I think it would be great to have more control over this feature. Being able to choose file type, image quality, etc. when pasting album art would be really beneficial. In order to maintain a consistent size, it would also be cool if pasting or otherwise embedding album art allowed you to set a maximum image size which MusicBee will automatically scale larger images to, similar to the same option you're given when synching music to a device.
All of that being said, I don't know anything about coding or creating a program, so I have no idea if these suggestions are even possible or just a pipe dream. However, I thought it was worth suggesting just in case it is something that can be considered.