I'm brainstorming a model for server clustering so that we can maintain a good ten year lead on ActivityPub. This is sort of like "server blocking" but a completely different paradigm. Basically your server can join 'clusters'; which can be public or private. This is sort of a different take on 'directory realms' where topical communities (for instance church groups) can restrict federation to those within their cluster/clusters. Any server admin can create a cluster, and these can be public or private. If it's private, a site must request membership. from the cluster creator. In either case a server can be removed from a cluster. There is always one default public cluster that includes the whole network and which you will be connected with by default (as a bootstrapping phase for new servers) - but you are free to leave it.
As a matter of policy, the public stream will only include servers from your site's chosen clusters. Some serious questions remain - can an individual site member connect with (or even see) somebody outside their site's chosen clusters? Should threads be collapsed by default if they originated outside your cluster and somebody within your cluster commented on them?
Also this could change the entire directory model. A site that creates a cluster would probably end up serving the directory for that cluster and we would need a model for continuance if the root cluster node or admin went offline. The directory page would let you see directory contents from different clusters (those that your site is a member of).
What I've just described is an initial brainstorm and a lot of things could change. Before I proceed any further with this idea I'd like to solicit opinions on how you see something like this working best for you - or perhaps it doesn't work at all for you.