I guess the reason nobody is doing anything with federated communities is that they don't really know what it means or how to go about it. Let's change that.
A community is nothing more than groups of people with aligned interests. There are a number of ways you can create these structures in the fediverse. One is through groups. This is the most popular. So is organising by "following" hashtags. This has a number of flaws, but was advocated by Diaspora some years ago and has a number of adherents. With the rise of Mastodon a few years ago, we saw the emergence of communities as themed websites.
We support all of these, but we're also doing something which enhances the possibilities in some new and different ways. That is "aligned/connected websites" or federated communities.
Here's how that works...
Joe runs boomchakalaka.foo and Fred runs nottyornice.foo and they discover their users are closely aligned. So let's connect these website communities into a new community called "spunjbobber". Joe visits the channel manager and switches to the "site channel" and changes his site name to spunjbobber. So does Fred. Now if you look at the Communities app you'll find a new community type called "spunjbobber" containing two sites. We're almost there. Now Joe (still connected to the site channel) connects to https://nottyornice.foo.
This connects one site channel to the other. Fred does the same (or just approves the connection). Now these two sites will share all their public posts and both will be visible in each other's public stream (in addition to or instead of any other sites that they permit in their public stream). So both sites have become a federated community and the members and content of each are more discoverable to each other than if they were isolated websites.
Want to follow a community without joining it? Anybody can follow the site channel of any site or community and (if approved) receive the stream of public posts from that site or community. So if you had a bicycle community containing 6 different sites, one could follow all of these sites as a single community and get the public posts from all of them. This last step isn't yet implemented (it's possible to connect manually to each but not yet automatically to all), but that's where we're going.
That's the high level summary. There are still a few rough edges but the infrastructure for this has been evolving over the last year or two in the (streams) repository and is usable today. You're welcome to improve on it or make suggestions or adapt it for your own needs.