Pretty much in order of priority for me. The last one would not significantly simplify my music cataloging or use of while the first one would for sure.
1. Ability to create a slide show and make it unique to the playback of an album first and foremost, but a playlist would be good to. Clearly, you can set the playback display to be from the images in the library. But I'd like to be able to assign a set of images or maybe even video to the songs of an album, sort of like multiple album covers for one album, that rotate when the songs from that album are playing and then it switches to a different set for a different album. I have some recordings where I've scanned the several pages in the booklet insert or I have photos of various aspects related to the recording (particularly live recordings).
2. This one has much support that I've seen in posts. The way it dawned on me was - similar to others - that I go through considerable organizational and data input effort to utilize the standard and custom tagging of different files, only to not be able to see that data in the view that is most conducive to 'entertaining' playback - Theater View. My specific suggestion is Ability to control the views in Theater View to be controlled via the same "View Settings > [sub-menus]" settings I can set for Playlists/Smartlists folders, etc...
3. Ability to have the 'auto-fill' of manual entry into tag fields work in the "Edit Disc Information" screen that comes up when the disc information cannot be found. Inevitably, my workaround is to use the Tag screen from the action window, but that means canceling out of the process that occurs when you have auto-rip turned on.
3a. Would be great if the auto-fill worked for the "Name" tag for song titles, for example. I'm routinely typing in the same song title completely in those instances where I have to manually enter the data.
4. Ability to set default album covers by artist. I created an album cover for Unknown Artist / Unknown Album and I noticed that in those situations MC automatically assigns it when the CD cannot be found on the internets. Would like to be able to use "Quick Find" for only the artist to set the default image by artist.