I think this would make QA much more powerful and users more comfortable with their custom design. For example, ckeditor is great but consumes about half the page size causing significant slow loading. For users who prefer it, they can still keep it while other users might recommend to choose Markdown editor.
Of course this modification can be considered during loading so that non usable tools/scripts will be skipped to save page size and loading time.
Themes as well change according to user's mood.