Since FerretDB is built on top of Postgres, could it be an extension? So that one can mix some FerretDB tables and some general SQL tables in the same database (with proper foreign key constraints between them etc)
What about optionally validating some columns with jsonschema? Perhaps using https://github.com/supabase/pg_jsonschema - is using other postgres extensions supported in FerretDB? (if not, maybe it's feasible to incorporate the code of pg_jsonschema in FerretDB?)