When I was looking around whether something like this already existed, I could only find some commercial (and discontinued) attempts, but nothing based on open standards. So I made a draft of what I think an open single-file music album format could look like. It is very simple, and basically just a specification of how to combine several already existing formats into one file. It is also compatible with the ZIP files you get when downloading a FLAC album from Bandcamp.
While some players (Foobar2000, DeaDBeeF) should already be able to play this format, I also made a cross-platform (desktop) demo player that fully supports the suggested format (including the digital booklet).
I am aware that this is not going to gain a lot of traction, but I am sharing this as it might still be useful to individuals.