With malleability, I think you're talking about low-level control, where you work directly in terms of the data structures that will be serialized as JSON. You might be translating between the domain data structures and the JSON structure; or they might appear direction as JSON. This is malleable in that you tweak it however you want; and it's simple in that you have direct access to everything. You can do validation in the same way. If something goes wrong, you have all the information available to deal with it as you see fit.

The wire format doesn't affect this approach - it could be JSON or XML. However, JSON and data structures maps cleanly, because it's an object format already. To do the same thing with XML requires an extra level, and you get a meta-format like xmlrpc, which is pretty ugly.

So I think you're talking about a kind of object serialization, with object-to-object data binding.

XML Schema is an attempt to factor out the grammar of the data structures, so that they can be checked automatically, and other grammar-based tasks can be automated. I think this is a worthy quest, succeed or fail. One specific failing we discussed was error messages.

I'm trying to grasp your point of view, and presenting what I think it is, so you tell me if I got it right or not (assuming you see this reply).