In my opinion, there's not necessary any payload format (json, xml, multipart). In keeping stuff KISS, everything we need is just an address in response + potentially some stuff like HTTP redirects (for providing additional compatibility for proposal of bitcoin URIs with "amount", "label" and other parts). I don't see reason why we need some extra payload yet.
If the idea is to "KISS", and provide a method that is both quick and
easy to implement for the average developer, then JSON is a stand out
option. Using HTTP for the data interchange will make things difficult
for a lot of developers if multipart responses are used. JSON will be
greeted with open arms.