have you seen http://json-schema.org ?
Yes indeed I have, and I have several problems with it, one of which is "Expires: August 3, 2013" with no new version in sight. My other major problem is that AFAICT it doesn't support one of my pet favorite features namely "Algebraic Data Types"[0, 1]. If you extend json-schema like Swagger has done it might support ADTs properly AFAICT, but I don't have any actual practical experience with Swagger.
[0] https://en.wikipedia.org/wiki/Algebraic_data_type [1] I should note that support for ADTs in XSD is sometimes sketchy in the various code generators, but at least the XSD specification supports it. (It could also be argued that XSD supports something even more general... which it probably shouldn't since ADTs basically cover the whole data structure space unless you go to higher kinds, inheritance and such.)
(1) lots of general tooling support, in particular you get at least decent editor support for your config file, and
(2) you can autogenerate the code needed to read your configuration into structured data without having to do any unnecessary duplication of "key names" (tags) as you have to with e.g. INI or JSON. You also get the data sturctures themselves for "free" (based on the XSD).
Alright, it's not the end of the world to not have these things, but they're both very nice to have.