I am developing features and found it frustrating a feature property can not be configured using a list or object by design because they were excluded from the schema. I can't see any rationale for not allowing the full scope of types for development. If you are concerned about creating an interface for selection, as has been done for VSCode, I consider that an implementation detail as devcontainer is meant to be a format with flexibility outside this context.
JSON is a great format for configuration but not at all pretty or user friendly when you have to triple escape quotes to pass an object in as a string because the JSON is already evaluated. Can you please enable lists and objects in the feature schema so I don't have to resort to triple escaping which is ugly and prone to errors.