This is a really great idea. However, it's incredibly complex. To allow anyone to do anything with the editor via the Plugin SDK, we have to re-architect the whole editor to be pluggable. That also means a huge amount more testing and bug fixes since the editor has to support usage scenarios I've never thought of, which is tough to get right. Finally, the SDK being in Javascript doesn't lend itself well to this, since we would basically have to write a whole Windows UI-to-Javascript layer... it would probably be more practical to make a C++ editor-plugin SDK. However this is a lot of work and we just don't really have the time to spend on it for now. I hope to return to something along these lines in future though.