This feature has already been mentioned by a number of others, but I am adding some further thoughts on it.
It has been mentioned a number of times that Roon should show more of the album artwork - if not automatically scraped, then what the user has downloaded and put in the album folder.
I wonder if on the main album screen, the existing stuff could be shifted to the left a bit, with a space for the rear cover art to the right of the text. I think this would look good, as the front and rear artwork would then “frame” the album info and the description in the middle. If there was no rear cover art available, the layout would default to something like it is now (or the new layout that is planned). As always, this could be an option rather than enforced.
Another idea is to have very small thumbnails below the front cover of the other scans or images that the user has in the directory for that album (or perhaps the parent directory, if it is a multi-CD set). The user could then click on one of these, which would cause the front cover image to be replaced with the image the user clicked on; or perhaps even better, have the image expanded, such as when you click on the front cover. Once expanded, the user could move left and right to go between the available images, as if they were leafing through the booklet itself. You would almost be able to smell the paper. When the users leaves the album screen, the image would default to the front cover again.