[temprature]

The Signal server software _does_ federate. It has had federation support since the first commit in the git history. Whisper Systems' server federated with Cyanogen's server for a while. Moxie has said it was a disaster and that is part of why the official Signal server won't federate.

You can, right now, run your own server and get other people to also run their own servers. Fork the Signal app, modify to ask for a server to use, and try and get people to use it instead of (or as well as) Signal itself.