SX already validates inherited vocabularies.
There is an ISO standard for declaring, in an XML resource, conformance to one or more inheritable XML vocabularies. (In the ISO context, such a vocabulary is called an "inheritable information architecture".)
Vocabularies can inherit from other vocabularies.
A single XML resource can inherit from more than one vocabulary.
Vocabularies are expressed using ordinary DTD syntax (with minor, optional enhancements).
Demonstration using the Topic Map inheritable vocabulary.