In order to provide the possibility to use different back end different of libxml2, I think is very convenient to change most of classes to Interfaces. Please consider the actual implementation at serializable branch where we had two implementations and is up to the user to select the most fits his requirements.
This implies to transform Node and most other like Document, Attr, and others, as GInterface and create classes implementing them using libxml2.
In the future some efforts like write Readers/Writers based on Vala or others could be used with the same API.