NIP 65! But really, anything to do with indexing and discoverability. NIP 89 is also really useful for helping people find special purpose clients, and NIP 01's relay hints are super important for helping people who are different relays find the content you're pointing at.

I think we've only really begun to discover solutions to the problem of network partitioning, there is a lot of research still to be done in this area. The fact that kind 10002 is mutable means that you can't easily look up people's relay selections at a point in time. Relay hints are set in stone, but easily become stale. I don't know what a decentralized indexing solution would look like, but this is part of what I mean when I talk about "de-commodifying relays". Relays should have different data sets, and reconciling/navigating them should be core to what nostr does.