If they ever did support federation I think it would make sense to have it include some level of coordinated service level agreements and vouching to avoid people signing up for random servers that someone then pulls the plug on. Also considering the trust you would need to extend to each server you use in terms of security updates, privacy and additional potential for MITM attacks. But given Moxie's lack of enthusiasm I doubt it'll be happening anytime soon.
I wonder if it would be possible to have an user migration between servers without cooperation from the origin server. This would allow users to move to a new one without losing track of existing conversations.
Seems crazy, but the reason we can't do this with email is the lack of a generally agreed identity for an user account that does not depend on the server itself. Signal accounts have a "master" key that can provide this and it's only stored in the device and backups (it's the most trusted of all keys, after all).
A sketch:
- User creates an initial account on server X (account: user@serverX.org), the procedure includes signing a message saying "I use server X since $TIMESTAMP and this is the 1st server that I use";
- Everything works as now.
- User wants to change server, so they signs a new message "I use server Y since $TIMESTAMP and this is the 2nd server that I use" (account: user@serverY.org); this message is sent to all chats/groups/contacts and to the old server (as an information only, it may be already down or be non-cooperative). Contacts update the server part of the account and start sending messages through the new one. Maybe the user can still try to contact the old server for a while, for the event it delivers a message from a account that didn't get the first, but at some moment all users will get the new address.
- Notice: I have no idea of how this can work with sealed senders of other metadata-prevention measures that Signal uses and we all love.
