Along the same lines there is talk about discourse and nextcloud extensions which also have large installed bases and amenable to single-click type installs.
In a sense any self-hosted server that publishes stuff is a candidate to get integrated into the fediborg.
What would be really sweet is to pre-emptively embed activitypub federation into generic platforms like django, phoenix, laravel etc. Right now various teams work in uncoordinated projects all implementing similar functionality.