some comments:

regarding message format I am more keen in it to be flexible, attaching on it "any" pair that a developer thinks it's appropriate instead of pushing a specific message format.  From the client perspective and in the future platform specific "client" frameworks we can hide this genericness by providing both platform specific methods e.g notification.addSound().addBadge() as well as addCustom("my-custom-key") and build out our key-value pair message format.

Thanks

Christos