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.
Creating a custom object and transforming it into JSON involves additional overhead due to object instantiation and property setting. For high-volume web API calls, this could lead to performance bottlenecks.