The most impactful use of ZKPs would be converting existing registries of various types (particularly, government registries) to be privacy-preserving (under specified conditions) and verifiable (granularly)

Better experience and privacy for billions of people, and simultaneously more efficient law enforcement

Not a gun guy but one of my pet fantasies is using ZKPs in this manner for gun registries, allowing people to purchase firearms compliantly while maximizing privacy. Feel like there would be a lot of PMF with that crowd.