I like the approaches in Pandoc markdown, and you are right, it would be great to have a settled format. Pandoc is nice and logical, but in the end I think it makes sense to choose a format which is already widely accepted by developers in the git{hub|lab} world.
Another thing that will count is the availability of an editor. I have recently bumped into this one: https://github.com/cesardeazevedo/nostr-editor
If it would be easy to drop it in a web app without hassle, that would lower the barrier of entry for markdown in apps, and probably drive motivation to standardize.
I will play around with the editor soon.