Now that we have serializer in core, and being used for JSON-LD, it makes sense to provide support for JSON and XML too. I think all we should need to do is register the encoders and normalizers for each in the container. I have included a very basic generic normalizer that returns the getProperties() method from an entityNG class. We then get support for these formats anytime we use serializer, such as : #1819760: Add a REST export display plugin and serializer integration., which uses a generic serializer plugin.
Initial patch attached.