One of the things I personally would love to see in BookBrainz is a way to track progress of things I’m reading, preferably with a possibility of keeping a history of progress updates and maybe even per-update comments (e.g., similar to https://www.goodreads.com/review/show/5254177026 ).

This would be somewhat ListenBrainz-type data and might be more appropriate for CritiqueBrainz too, but right now this functionality is nowhere at all and is definitely one of the main things keeping me on sites like Goodreads.