The killer app for markdown would be a collaborative editor that displays the raw markdown and formatted markdown side-by-side and makes both sides editable. Tech people can use `#` and `*` on one side for formatting, product people can use normal text-editor buttons like "header1", "italics", etc.
IIRC the trick was to get a pipeline for Markdown to HTML, render it into a WYSIWYG editor, then convert the HTML to an AST, and walk that to generate the markdown. I had to “format” both the markdown and html on each render (bidirectional round trip render) because parsing/gen wasn’t whitespace stable.