For tracks/releases/artists, the response to ‘get library item metadata’ (/library/metadata/{ratingKey}) includes the item’s Musicbrainz ID (MBID) in a field called Guid, if it exists. It would helpful to also include the MBIDs of the parent/grandparent items in this response, as well as in the response to ‘get playlist items’ (/playlists/{playlistId}/items).
At the moment, if I want to find the MBIDs of every track, release, and artist in a playlist, it’s necessary to make 3 API calls per playlist item (one ‘get library item metadata’ call each per track, release, and artist). Providing the data in a single response would drastically reduce the number of required API calls.