I’m delighted to see this project evolving and look forward to following its development! I’ve used just about every markup technology over my (many) years as a tech writer, from troff macros, to SGML/DocBook and then XML/DITA, and finally, to Markup with the Material for MkDocs project, as a sponsor. Each has its strengths and weaknesses, but for enabling contributors outside the tech writing community, simpler source formats are the way.
That said, if pressed, I’d recommend AsciiDoc[0] over any Markup flavor for a greenfield project _today_. We had to either add or bake plugins or extensions to get features that are already included in AsciiDoc, making our Markup implementation both more complex and wholly unique. That wasn’t a huge problem, because we didn’t have a large pool of contributors to educate and support, but it would have been much easier just to point to a standard.
But, hey! The roadmap includes modules, to make way for other source formats! This is the way. :-)
[0] https://asciidoc.org/
EDIT: s/That's the way/This is the way. :-)/