JSON schema exists. If you restrict yourself to a sensible subset of XML features in your XML schema, you can have a 1:1 correspondence to JSON and JSON Schema. We do that at work. Due to historical reasons we have a XSD but provide the complimentary JSON Schema to those who wish to send us JSON.

The JSON is converted on the fly to XML based on the XSD so it can be ingested by our existing XML integration. Similar with return answers, response XMLs are converted on the fly based on the XSD to JSON.

JSON endpoints validate against the JSON Schema, also generated from the XSD at runtime, XML against the XSD of course.

We had a diverse set of XSDs but didn't have to tweak them to support JSON. We used restrictions and extensions, both simple and complex, we used min/max, enums, descriptions and examples and more, so not entirely boring XSDs.

We did establish some conventions, attributes turns the child element to an object and the attributes become properies, just simple stuff like that.

This way customers can hand us what they prefer generating and ingesting, and we don't have to worry about keeping two different schemas for the same endpoint in sync.