Ideas/Requests -
A way to launch a exe (MMF2 made also hopefully), jar or webview containing a swf or javascript from the edittime side of things would be really useful. What i have in mind for a extension is a editor that you generate a string output with, so it doesn't need a return to work as you would just paste it into the extensions parameter input in a action etc.
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.
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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.
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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.