Just letting everyone know I opened this up as a contest for general interest:
I still intend to finish the written CommonMark guide at least. An accompanying interactive guide (or as part of the existing guide) would be useful, either just a simple “try it out” editor added to each page, or something fancier. It will be interesting to see what comes out of this contest.
Code guides complete. Just the Escaping Markdown and HTML guides to go (for the long form guides).