Haven't seen a NIP for this yet, but I'd love to see some proposal for reputation/web of trust. This could be very useful for and sort of peer-to-peer trading or interaction, especially if we can get different algorithms for figuring out how much to trust someone. Might need a different way to relate than follow, though.
Discussion
I feel this might already be possible using things like replies and likes, which are kind of like implicit reputation in a way. Someone would just have to figure out how to measure it from those things.
Here's an off-the-cuff idea. Before a peer-to-peer trade both Alice and Bob send a signed message to each other. Alice signing "I have initiated a trade X for Y with Bob". Then after the trade, Bob countersigns the message with "I successfully completed this trade with Alice" or "Alice didn't live up to her end of the bargain." These can be tallied up based on how close they are to you on the network for a score.
Web of Trust could be the basis of significant efficiency and social connectivity for improvements for nostr relays. Currently relays do not relay to each other. Instead users must post to multiple relays with the hope other users sharing the same relay will see it. The discoverability situation is already tenuous: e.g. I see users who share just one common relay out of my 17. This has long-term scaling implications because many-to-many connections do not scale.
Web of Trust could form a backbone to know which notes are safe to replicate between the relays. That would improve discoverability for a lot of people without every user needing to share the same relays.