Ability to set Namespace mapper in the JAXBContext instead of Marshaller.
The Marshaller is not Thread safe and hence it is recommended to create a new instance while working with it. JAXBContext is however thread-safe and is recommended to keep a singleton for improved performance.
With this in mind, I think a feature of being able to specify namespace mapper at a JAXBContext level would be preferrable.
Even otherwise, most of time, users would want to use same namespace mapping irrespective of how many times the marshall contents to XML document.