What I am talking about is instead of recreating jsFiddle in the dev tools, how about letting jsFiddle (with appropriate user confirmation) control the dev tools?
As the creator of Plunker (http://beta.plnkr.co/edit/e5iLyQ?p=preview), I would love to be able to leverage the environment that has already been created to allow adding breakpoints and inspecting of running code. Since Plunker already has live-previewing and a working multi-file editing interface, it would be really amazing to be able to take it to the next level.
Shameless plug: If you've never heard of Plunker, check it out. For free you get live-reload of previews, transparent compilation (and sourcemapping) of stuff like Coffee-Script, Typescript, Stylus, etc.., real-time collaboration on the same code, live js/css linting and much more! Also, it is open-source (http://github.com/filearts/).