It occurs to me that a lot of the requests on sharing notebooks could be handled simply by the client being able to handle multiple, completely independent accounts with their own synchronizatoin sources. For example, a personal NextCloud account and a NextCloud account that is already shared with other people (e.g. a team NextCloud account, or one used with a partner).
My pressing need at the moment is having separate team notebooks from personal notebooks on our university's webDAV server (which we're stuck using for medical data protection reasons), and it feels like I have to use Joplin either for one or for the other, but having multiple accounts would totally solve that. For us, the desktop client is the most valuable one, but I'm sure this is a feature that would be useful on the mobile clients as well. If we can share notebooks or collections of notebooks that's great, but it would be fine if we just had a team webDAV account that we added to Joplin.
I hope others also think this is a useful idea.