So Google's protocol buffers have this feature called "required" fields, which enforce schema in the type system. You should never use it. Never. It's one of those things that sound good until you're a few years into the project. Similar to how you should never be using meaningful IDs as primary keys for objects, always use meaningless fingerprint-like integers. Or how all integers should be signed unless you're dead sure the number is unsigned (like a fingerprint). And how many integers should actually be strings, unless you're dead sure this is a number (externally provided IDs, such as for example customer account IDs, are not numbers). Or how you should be careful to use bytes rather than unicode strings.

Make your schema permissible and your code paranoid, it will pay off later. Build a data linter if necessary, but don't tie the schema.