JSON quacks a lot like a Python dict, so then why not Python (or Java) style dotted namespaces? An arbitrary bit of JSON might be safely extended with GeoJSON geometry like:
Or with a feature:
The GeoJSON working group rejected XML namespaces (such as in JDIL) for 1.0. I recall that I had the most strongly expressed opinion: that GeoJSON should be distinctly JSON and not XML without angle brackets. JSON is less abstracted than XML, closer to code, and dotted namespaces seem like a win to me.