Clearly coordination and agreement between leaders is needed. I suggested something like that with a pseudo-instance of "@global", for example. However, it seems there is some resistance to the mere idea of globalizing certain popular communities, which I can understand.

Federation comes with its own set of problems, like replication, data volume, storage requirements, and massive overlap.

That last one affects user experience directly, and needs to be addressed. Maybe it will sort itself out, maybe not. If we have 10,000 servers, even 100 almost the same communities means quite a bit of work on the part of users just to decide which ones to join.

We are looking at the human equivalent of a system with an extremely fragmented disk, or database tables with indexes that end up doing table scans.

Periodic re-organization will be necessary to to maintain usability.