Colloquially Unity 2,3,4,5 are just called Unity. Same goes for UDK which they are on to four. I suspect when Construct get's to 3/4 people will just call it Construct. However C2 exist to separate from C.Classic. That's because C2 is a major engine update from C++ to JS.
I suspect that internally the exporters are probably de-coupled already. Your post count is 3 so I suspect you have been lurking. But C2 was going to support native Windows first, and a poll people voted HTML. For whatever reason it's only JS. But I will point out that the problem with exporting isn't the exporting. It's that all the Plugins are written in HTML5 JS, and not generic JS. I suspect if the plugins where written in V8 JS and used an abstraction layer for rendering, audio, asset loading so on so forth. Then JS could be compiled to CLI and that could then be compiled to other systems. However since the Plugins are embeded and reliant on HTML Browser features(Browser XML, WebSocket, WebAudio....) that there is just no way to support other platforforms without breaking the one set of plugins flow. Personally I'm up for platform independence, and then let othr programmers do all the other platform exports. But I gave up that request 2 years ago
I'm pushing for the new IDE to be written in JS for NW.js.
I'd like the option too for Plugins to be in packages. That way CAPX will also carry the required plugins. So that if your getting a CAPX from another. The plugins put into the CAPX/Plugin folder go with it. We will see.
I think you missed the JS SDK to write your own plugins. If your talking about writing code in files. I did that. I wrote a behaviour that let's you attach code object/file to an Object. Works 90% great. I just haven't figured out how to get the file to be compiled by the JIT ... I think the files may need manual insertion. I plan to present this plugin to Ashley when it's more feature ready.....
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though I'm kinda liking the C#/Java compile idea to JS. I can dig that. Heavier types language to reduce errors. That would kinda awesome.