Apparently, Asciidoc is getting an open specification under the umbrella of the Eclipse Foundation.
It always had some features by default that Markdown only has in (often diverging) extended flavors, also some design choices (e.g. intraword emphasis) are better than in Commonmark (mostly for legacy reasons), but I really dislike how Asciidoc relies on English keywords for some key features like links.
Asciidoc is nice in many ways. But it prioritizes parsing simplicity over readability; for an illustration see the example in section 1.1 of the Commonmark spec.
I wish them luck on the spec. I suspect it will be a much easier project than for Commonmark.