This is a really cool project. I've often wanted to revive .plan somehow but never thought about doing so over http. Markdown makes a lot of sense here as well. Good job.
Thanks! I think the next step is allowing the app to be a middleware, distributed via npm, so that adding a .plan to any project is as easy as app.get('/', plan);
And re: markdown, the thing that makes the most sense to me about it is that markdown is plaintext, and it's styling is not that much different than what one would actually do in a .plan file to begin with.