In practice this would be difficult to implement because each instance has its own take on how to shape the code for their site. There's no obligation to create an instance so that it will be compatible with everyone else's instance, and in fact I would guess that would be effectively impossible.

Let's say Instance A allows porn, and a user on A wants to create an account on Instance B, but Instance B doesn't want any porn on their server. At minimum, a way to keep any porn on that user's account from syncing to B's server would have to be implemented.

This is only a single case. There will be plenty more small issues like that to have to work around, so it will take a lot of time to get all that logic designed, implemented and tested.

The cloning of an account might also involve a not-insignificant amount of data being transferred. What if the receiving server wants to limit the amount of data storage for a new account so that they're not burdened with storing tons of data for new, unknown users? How do you then determine what subset of that user's data to import?

Maybe these things will happen with enough time, but for now I think it's best for now at least if everyone thinks of each instance as its own separate website that can communicate with other similar sites rather than a set of cloned sites where which one you pick doesn't matter.

Please don't take this as argumentative, as we need people to share ideas like yours! I just keep seeing messages that give me the impression that people have expectations for the Threadiverse that aren't currently realistic given what the state of the software is now.