Almost all my writing starts out as Markdown for all the usual reasons - portability, vendor independence, easily converted to other formats in automateable ways.
Increasingly, though, my technical writing ends up in AsciiDoc syntax for publication, both in print analog like PDF or on the web. The good news is that Markdown fully converts to AsciiDoc, and there are good tools that can be used in a publication CI chain to do the conversion. A typical workflow for me would be to create notes and snippets in Markdown using Drafts, filing those in DT, organizing the snippets into a chapter / paragraph structure in DT (sometimes with a trip into MindNode to create a structure), and then converting the chapter to AsciiDoc and putting it into the content management CI workflow.
So, I’ll suggest AsciiDoc as a dark horse format candidate. I’d also be interested in knowing if others in this Forum are using AsciiDoc.