This is an interesting idea, the goal would be to have syntax highlighting in the textarea (i.e. the left side of the composer) so you could see if you’re making any markdown syntax mistakes. Yes, code blocks do the same in the preview, but you won’t see any markdown mistakes, for example. And some complex markdown would be easier to scan, post with lots of tables, uploads, links, headings.
I’m not sure it’s easy to do this, though. Our admin screens use the ACE Editor, and I don’t think we can just drag and drop that on the post composer.