Proof of Person is the problem we most need to solve.

A means to cryptographically assert in a privacy-preserving way "a human being generated this"; or less privacy-preserving, "human being X generated this".

To do either in a distributed fashion would perhaps involve peer-to-peer attestations of human-ness, published as a "web of trust" in a distributed database. But any anonymity/pseudonymity would be easily broken if done naively.

Of course, "human being X generated this" is something that would be easily facilitated by governments, with certificates issued to each citizen/national. Americans can only dream of this happening at the federal level---states will have to be where the action's at.