In some sense, JSON Schema does lean in that direction. In JSON Schema anything is valid except what you constrain. For example, the schema {} will accept any v

In some sense, JSON Schema does lean in that direction. In JSON Schema anything is valid except what you constrain. For example, the schema {} will accept any valid JSON. Anything you add to the schema is kind of like a test that further constraints the schema. If I say {"type": "object"}, now my schema will accept any valid object. If I add {"type": "object", "properties": {"foo": "string"}}, now if there is a property "foo", it must have type string. I can still have whatever other properties I want of whatever type.