If you could launch local hosted apps and have returns back to propeties or if not directly through a string reader or something that would be even better though since i can make nice editors 100x faster with MMF2 than i can with C++ code.
A way to have one control switch values within the JSON or by reading a local txt would be really cool. Basically i am thinking along the lines of presets for a objects properties, so if it was a particle maker extension then you could switch between the different effects simply by changing a dropdown lists item etc. Hopefully this would be possible also as it might be similar to the multi-language setup you describe.
You could maybe have a automatic documentation generator which reads the JSON and outputs a HTML template etc ready to edit. I notice some developers don't make docs but if most of the process is automated and they just need to add descriptions manually it might change that.