Right now, the various BSON specific types (e.g. ObjectId) will serialize to / deserialize from the extended JSON representation (or its equivalent if not in JSON) of the type when not serializing to BSON. This can sometimes be surprising to users that expect more friendly formats, such as a hex string for ObjectId or an ISO-8601 formatted string for DateTime. We should provide a way to specify alternative default serialization formats through a feature flag (or some other mechanism).