There was a previous discussion here, about how a feature similar to multi-reddit can be created in fediverse.
https://mander.xyz/comment/271745
I think that if each community can be associated with some tags, then users can just subscribe to the tags, and follow feeds from all related communities. Some more details can be added to this idea, for example, posts can inherit the tags of a community by default, and the poster can customize the tags for each post. Other users can report some tags if they are incorrect. Each community and user can have a tag health, which would be high if their post tags are not reported too much, and then those posts would be shown to users who subscribed to those tags.
Maybe this can create a feature similar to multi-reddit.