AFAIK, the only practical thing in the way of having a separate server that just hosts identity accounts for all types of fediverse content (while the content itself is hosted on other servers) is that your host server is responsible for presenting the interface through which you view the rest of the fediverse, and the interfaces are specialized for a particular content type. You could have a server running a variety of fediverse software (mastodon, lemmy, etc.) which automatically generates similar accounts for each user on each service, so users could sign up once and then switch interfaces; but I think the rest of the fediverse would still treat them as separate identities.
jelloeater85