Hmmm what if there was a new fediverse service dedicated to hosting fediverse accounts. You would choose an instance of that service to create an account and then use that account to register with a fediverse instance of Lemmy, Mastodon, etc. Kind of like you can login to various websites with your Google or Facebook account.
That would mean a single account could be used with any Fediverse service (Lemmy, Mastodon, etc.), all post history could be aggregated, and you could easily move from one instance to another.
Lemmy, Mastodon, etc., would have to be enhanced to accept this mode of registration of course.