I think this could be a much more elegant solution than MassMessage. I'm not a huge fan of the way MassMessage adds a new section to my talk page, and I think being able to send out Echo notifications to the users subscribed to a Newsletter, WikiProject, etc, would be far better than the current Talk page system, especially since Flow will eventually render the MassMessage system unusable.
As a manager of a small WikiProject task force, the ability to provide something like monthly updates on the progress of the project would be greatly appreciated. I'm sure the writers of The Signpost and other newsletters would be similarly appreciative of such functionality, not to mention the ability to easily get feedback on new features (such as Flow or VisualEditor) from interested parties.
I also think creating an official system to join/subscribe to things like a Newsletter or WikiProject that doesn't require editing a page and has a singular central hub for managing subscriptions/membership would be absolutely fantastic for the productivity of all users and projects.