Creator of https://plnkr.co here. Amazing work on StackBlitz. Looking forward to figuring out how it all hangs together. Very novel approach indeed.

Anecdotally, I was the one who contributed ?json support to unpkg for a related use-case (in browser editor). OSS at its best, moving everyone forward. I use use the feature to provide a similar (but more limited) package injection. Check out https://next.plnkr.co/edit/ (book icon in toolbar of html panes) to see it in action.