That's where my federated Reddit alternative idea would come in. Each forum would be hosted by the community that uses it, and the centralised site would only act as a feed for them with a shared login system.

Something like that seems like the next logical step after Reddit, and the way to make separate forums the main way of talking online again.

That's where some sort of syndication feed would come in, like RSS or ActivityPub.

Doesn't the err, multireddit (?) system work for that? I recall that was a thing on the old one at least.

The multireddit system doesn't work outside of reddit.