I am currently working on a custom markup language called atex. It's syntax reminds of latex syntax, but with @ instead \ as a special character (very similar to the Lout language, if anyone remembers it). Also, the atex language hasn't any predefined commands. Instead, all commands are defined via schema specified in a separate YAML file. Schema defines commands that can be used and means of "rendering" those commands to different targets (HTML, Tex, Typst, whatever...)
Just today, I finished first working version of the new compiler (https://github.com/ubavic/mint). It is written in Go, and there are lot of things on the TODO list, but it works :)
This is actually the second compiler for the atex. The first one was written in Haskell and compiled fixed document schema. I used it for writing a book on Haskell (https://github.com/ubavic/programming-in-haskell).