Deeper intergration with p2p file sharing\streaming-oriented sites and protocols, e.g. torrents, but decentralized too. Think like Blizzard with their p2p acceleration of downloads using other people's downloaded game. Basically, making your own PC a source of content for those accesing instance. May probably ease traffic reqs for instances and make them more reliable under ddos.

Infrastructure for old-fashioned game servers' communities, like those for Minecraft. Login with your fediverse account, one-click joining server, creating federated networks of them with theoretical migration of progress\playtime\achievements. It can make this way of multiplayer gaming convinient again. Yet again, it needs specifically tailored games\clients.

Mapping and geocaching communities. A network of overlaying local map-canvases that people can interact with by adding favorite routes, comments, points of interest.

Books and articles shared between repositories of different universities\libraries.