Modeling message packages

There will be a hierarchy in the structure of message packages. An abstract Package class will provide a means by which all package sub-classes like XMLTransform, Render and maybe even sub-classes of Render like RIBRender or YafrayRender can inherit the basic properties and methods required to construct and manipulate a SOA-R message package. An alternative might be to use an Interface instead, but if I change the interface existing code may break. So I'll have to make sure I'm happy with it before I enshrine it in an interface. I have decided to call the class (or interface) a 'JobBag'. It would seem fit for the purpose. Now, do you suppose that by passing a load of metadata in the JobBag, that the message would derive any advantage from being expressed in RDF/XML rather than just plain XML? If I create my own WSDL then I'd be free to define the schema as I see fit. But am I just gilding the Lilly - so to speak?