The way I picture this is by letting communities have some sort of “partner communities” listing. If mods of games@xyz decide they like the content of games@abc, and gaming@123, they add those communities as “partners” (perhaps those communities have to accept which in turn adds games@abc as their partner). Then, when any user subscribes to one partnered community, they also become subscribed by proxy to the others, and begin to see posts from all 3.
This helps smaller communities piggyback on the success of willing larger communities and gain a bit of visibility as well, which should encourage growth of each partner so smaller ones don’t just die out.
Communities can “unpartner” at any time, in which case users would only remain subscribed to the one they originally selected. And of course, users could explicitly block any of the partnered communities if they don’t want to see the whole set.
Defederation blocks communication both ways, I believe.