To keep track of the limitations of the metadata API, here are some though on using it for the photos application. More might come later.
Extend WebDAV search to support searching, filtering and ordering with metadata. Example: if the taken date were extracted from photos' EXIF data, the photos app could request to order the timeline request with it.
Move metadata extraction to an asynchronous background job. Example: reverse geocoding can take some time and is better and would slow done response time of an upload of time synchronously
Metadata can depend on other metadata. Adding a priority or dependency logic for providers would help.
To keep track of the limitations of the metadata API, here are some though on using it for the photos application. More might come later.