For schema stability we need a place where we can store identifying information for schema itself and various configuration-related info as well. That will server as a sort of metadata for the rest of the schema and data in registry. To be usable from many releases it itself needs to be super-stable but we also should be be able to store all sorts of metadata in it, exact structure of that is not possible predict today. I think the most generic solution for that is a simple key-value store with arbitrary keys and values encoded as strings. All other structures can be mapped to key/value using reasonable rules, though some care is certainly needed to avoid name collision.