Instead, make codegen a function of BOTH a data schema object and a code template (eg expressed in Jinja2 template language - or ZeroMQ GSL where I first saw this approach). The codegen stage is then simply the application of the template to the data schema to produce a code artifact.
The templates are written assuming the data schema is provided following a meta-schema (eg JSON Schema for a data schema in JSON). One can develop, eg per-language templates to produce serialization code or intra-language converters between serialization forms (on wire) and application friendly forms. The extra effort to develop a template for a particular target is amortized as it will work across all data schemas that adhere to a common meta-schema.
The "codegen" stage can of course be given non "code" templates to produce, eg, reference documentation about the data schema in different formats like HTML, text, nroff/man, etc.