I have to say, I'm not liking this whole fat plugin idea. It's great from the point of the view of someone using the editor, as they can just install one DLL and have it automatically work in the engine, but the issue comes with distribution. When I go to distribute my game, do I really want to add bloat by including design-time code with the engine? Probably not. Let's not forget that more code is usually needed for the IDE side than the engine side...This is one of the reasons recent versions of Visual Studio actually separated the design-time components from the runtime stuff, to reduce the size of the redistributables.