I'm splitting the WMDS-specific details from the parent task.
Here's what I'd like to discuss at the developer summit for the Shadow namespaces RfC.
1. At what layer should shadow namespaces be integrated into the code? Should we continue to subclass Article? How do we efficiently implement things like batch lookups? (T88644)
2. How will we integrate remote content with search and other discovery mechanisms? We currently have Commons file results integrated with normal search results, but it's usefulness is questionable (T96535). Will it be possible to implement this for people using remote content via the API?
3. How will we keep track of usage and invalidate caches? Should we just have a central link table like GlobalUsage? What about API users?
4. Should we allow people to chain "repositories" like we currently allow with file repos?
The outcome I'd like to see is having a plan ready on how to migrate the foreign file system to a generic arbitrary namespace one, which should completely obsolete the #GlobalUserPage extension.