Much of modern application development depends on APIs. In the past we may have built massive monolithic applications, but modern cloud-native development depends on collections of microservices, linked by APIs that offer everything from remote procedure calls to database operations. This approach makes applications easier to run, allowing them to scale with user demand. It also makes them more reliable, handling failover in the event of data center outages.
You've already covered most of the "core" JSON utilities, so that's a really solid toolkit. A few things that come to mind that could make it even more useful: JSON → Markdown / Table generator: super handy for documentation and wikis. JSON Schema → Form generator: let people paste a schema and instantly get a web form to fill in.