IMO you can still add federation (and you should for freedom and security), but no need to expose it to the user. For example: set the default server to signal servers. For sake of sanity, you probably want to route all servers to the default signal server. But you could easily allow user@server. If you can figure out how to use phone numbers directly between servers, congratulations, cause it will not be easy. But I think people who want to run their own server will not find that much of an issue.<p>In terms of getting stuck in time, you could use though love, and require backwards compatibility for like a month but no-longer. But so long as you keep the phone-numbers @the signal servers, your server will be the reference, and any server not following will get the blame.