I would like to suggest adding federation feature to Joplin cloud/server to allow users to share notes to another user on another server. For example if john@joplincloud.com wants to share notes with Amy who was an account at amy@amyland.co, they can share notes between these two accounts.
Similar to how Nextcloud can share files with other users on other servers and federated platforms like Mastodon and Peertube.
Notes that are shared between servers must be encrypted in transit with E2EE.