Would each forum have the ability to host remote updates out of the box, so essentially it would be an alternate ‘view’? How would you cope with opening up Topics from this view? New tab per remote forum? Obviously there would need to be some kind of security and perhaps you would only receive fully ‘public’ Topics?
I think publishing of Discourse topics for remote follow is what should be aimed for.
Forum users following who-knows-what remote content should be an explicit non-goal, especially for the first versions. That’s not what Discourse is built to do; leave it to the generalist content viewers.
Discourse following of federated content feeds should be implemented in a way similar to the RSS plugin.
I would say the best way to describe might be ‘an aggregator of all chosen sources’, perhaps a Topic List with the Latest combined from n sources.
It’s open to debate what happens when you click on a topic. Perhaps, if the source is a remote forum, you are just redirected to the source forum immediately in a new tab. So you might argue it’s not a forum of forums as you are still deferring to each local forum to interact further.
This all comes down to a more detailed functional specification.