I've begun to wonder about this question. There's nothing implied in the existence of the fediverse that it will include every site, and there's nothing implied in the existence of every non-fediverse site which would stop them from forming their own fediverse. One could technically say email providers have already achieved this, though this only really applies to email providers which doesn't make it as useful as the fediverse we know and love. Are there actually other fediverses out there?
Tangentially related idea is how many sub-fediverse are their. Surly u could have 2 sets of instances that have all defederated the other group but are all federated with their own group.
Long ago Usenet and BBS networks worked in a manner that we could describe as federated, if you mean a decentralized system where servers could communicate with each other. I saw something a few months ago about a modern service that sounds kind of similar, but I don’t remember the name now. It seemed interesting but I put it in the back burner and then lost it.
If a few servers are linked up and talk to each other using TCP/IP (?) but aren't connected to the wider network, that's not enough for it to be considered another internet (but it could be an intranet).
If a few instances are linked up and talk to each other using ActivityPub but aren't connected to the wider network, I think that's not enough for it to be considered another fediverse.
There are hundreds of other federated systems, yes.
As per what your question is asking, I'm not sure if there are fedoverse systems branched off from the "main" one. I guess Lemmy/forum stuff can't really federate with Microblogging-style things.