OKF seems OK, but bound to Markdown. A Markdown document can be turned into an OKF document by adding a 'type' to the frontmatter YAML.

What about a knowledge graph language, which can be stated in Markdown prose, in Markdown code blocks, but also everywhere a text field is waiting for you? In the minimalistic language https://ddot.it you can link outside the Markdown world, to files, URLs or even just labels. Like OKF it's just a format.

Disclaimer: I wrote that (short) spec.

I love how unobtrusive that is, great compromise between readability and expressiveness!