Proposed Enhanced XML-RPC Standard
via Scripting News: A Proposal for an Expanded and Enhanced XML-RPC Standard (not by UserLand)
The enhancements Randy suggests look quite reasonable, he's trying to push logic out of the server handlers and user of the client, and into the transport (client and server). That means most (if not all) his goals could be achieved with more logic in the layers above the client and below the server. Not the best place for this kind of logic, but at least it wouldn't change the standard.
I'm really surprised Randy didn't include any new datatypes. NIL is a datatype supported by a few of the implementations already, and a LOT of people have begged for a tag that would let arbitrary XML tags into the request/response (without escaping them in a string datatype).