Description
I'd like to propose that the per-room privacy settings allow to declare that either any homeserver may participate or otherwise homeservers from a user definable whitelist are allowed.
My personal use case is that I need to be able to restrict certain rooms such that only accounts from my server can be in the room. (The implication being that the restricted rooms and their content do not get federated).
The generalization is that it is possible to define a whitelist of allowed homeservers. (In my case I allow only my own homeserver). Therefore federation can be limited per-room to whitelisted homeservers.
Currently the only way to prevent federation is to disable room invites for non-admins.
I think that having control over ones own data is one of the main selling points of matrix/riot. My proposal further strengthens privacy.