There is a schema for DFDL annotations.
There is also a schema for the subset of XML Schema used by DFDL.

The first thing Daffodil should do is validate against these. This will help reduce checking and standardize error reporting.

Obviously, this can only check what schema validation checks, so there are still plenty of checks to do in the schema parsing, and runtime, but doing this easy validation pass first before processing the schema at all, can only help reduce complexity.