I’ve discovered a new markup language called Typify Markup:
According to the website, Typify Markup is “a lightweight text markup language for the representation of structured data and semantic content.” That sounds a lot like the goals of CommonMark plus its proposed extensions.
I encourage contributors here to have a look at the spec, as it may be inspirational when devising new CommonMark syntax. What I like most about Typify Markup is the emphasis on readability.