The XML Schema for DFDL Schemas could be tightened up significantly so that many more of the constructs in XML Schema that DFDL schemas cannot use will be rejected up front when we validate the DFDL schemas using Xerces.

As an example, in DFDL complex type definitions cannot use derivations, nor the mixed attribute, nor block, nor final. Yet the XML Schema for DFDL schemas is currently allowing those. (They're likely just ignored, but should be rejected.)