The point of JSON is that it is completely dynamic and flexible. Bend it to whatever whim you would like, because it's just a set of serialized JavaScript objects and arrays, rooted in a single node.
What the type of the rootnode is is up to you, what it contains is up to you, whether you send metadata along with the response is up to you, whether you set the mime-type to application/json
or leave it as text/plain
is up to you (as long as you know how to handle the edge cases).
Build a lightweight schema that you like.
Personally, I've found that analytics-tracking and mp3/ogg serving and image-gallery serving and text-messaging and network-packets for online gaming, and blog-posts and blog-comments all have very different requirements in terms of what is sent and what is received and how they should be consumed.
So the last thing I'd want, when doing all of that, is to try to make each one conform to the same boilerplate standard, which is based on XML2.0 or somesuch.
That said, there's a lot to be said for using schemas which make sense to you and are well thought out.
Just read some API responses, note what you like, criticize what you don't, write those criticisms down and understand why they rub you the wrong way, and then think about how to apply what you learned to what you need.