My thought would be something like a small LDAP type server. Self-hostable. You make a user like myuser@mydomain.net and its honored as a log in for the various fediverse stuff.
So like it could hold the subsciptions for the communities on the various threadiverse servers you connect to localy, and when you open say lemmy.ml, part of the info sent for your user would be a list of communities you are subscibed to on lemmy.ml.
If it just handles the user auth, then it could also be a user auth for other fedivers stuff too. PixelFed, and Mastodon, etc. Each service could have its own sub section of the user object's info.
You would still probably end up with a "home" instance you would use, but if that home instance becomes untenable, or goes away, then you would just pick a new instance and log in there with your myuser@mydomain.net account.
Im not a good enough dev to code it, but thats my idea anyway.