I think themed social sites are the way to go for the fediverse, almost to the point where the theme doesn't matter. Any theme. Any raison d'etre beyond "to be a general interest clone of what already exists". So yeah, I think this is a good idea.
I think the suggestion also highlights some moderation/administration features that were missing when I first tinkered with self-hosting Lemmy a year ago. Are there tools to allow users to access these types of communities while keeping them hidden from the 'All' feed? There wasn't last year. It would be ideal to designate sites and communities that are A) totally blocked/banned, B) accessible/subscribable but only via direct url search, C) searchable, but not available in All (or even local, for hidden local communities), D) accessible via All. Or even having different discovery vectors selectable via binary selection. The fine grained filtering to do such a thing would be a real boon in general, especially for sites that want to remain thematically focused, while not handcuffing users who want to be able to view stuff that's off-topic.
Yeah, so no change from last year. That was a core reason I abandoned my exploration of Lemmy for use hosting my softball team discussion group. I couldn't prevent it from becoming polluted with my other community subscriptions.
It's a totally overlooked usecase, that I increasingly believe should be a core use case for the software.