GoodReads has a system whereby registered users can create mutually exclusive 'shelves', where books can only be in a single one of these shelves at a time. All accounts have 'to-read', 'read' and 'currently reading' mutually exclusive shelves.

It would be nice to extend this concept to MusicBrainz, so I can have a somewhat self-managing 'wishlist' and 'owned' pair of collections without having to remember to update both collections.