Great point. I'm revising it, and added a section on "Some hard problems – and some intriguing ideas" including

Consent-based federation is another area where today's tools leave plenty of room for improvement. Today's approaches to allow-list federation are very manual and cumbersome; how to make it work at "fediverse scale"? There are some tricky questions here, for example how to make sure that small and single-user instances don't get frozen out. There are intriguing ideas for other approaches ...