This seems like an easy problem to resolve. Revive the standards body for this protocol. Create a new group that will define the spec and schema, convince others to use this and you're good to go. Unless you need to remove elements, it should be pretty simple to expand on what's already there.

To me it doesn't make any sense to bemoan that it is in XML.. so what. It includes a schema and can be validated. That's a good thing. Don't throw json into this.