What about a plugin that copies the markdown into a collaborative online-editor, like Free Collaborative Markdown Editor - onlinemarkdowneditor.dev
after paste, one can simply copy the web-link and share it
(afterwards the plugin could maybe add the link to the note/front-matter?)
Of course this is not something for privacy-aware people, but it’s free! 
I tried to find other free collaborative markdown-editors, but couldn’t find one yet.
Besides this self-hosted solution: New Plugin: Sync to Etherpad Lite for easy web-based collaboration!