What it might actually look like ...
First and foremost, the extra load would cause 503 errors and duplicate posts.
would still be posts in threads in forums (ignoring the gallery section and the blog section and the articles section ... which might be simpler). It's not obvious that creating new threads from federated servers would work, as they'd have to identify the forum and provide a thread title. But replies to existing threads could work - you'd see user names like
@riikka@mstdnsuomi.fi instead of like
JoeP. On their end, they would see threads and posts on
as statuses and replies or some such, depending on how they present things.
In the reverse, federation would in theory allow us on
to see activity on other servers in some kind of swamp of unstructured timeline chaos ... possibly grouped under some other forum title ... "Goths, Visigoths and Huns" say.
But we wouldn't have to allow a free-for-all - you can choose to federate to some servers only, or block any you don't like.