At face value, the idea feels reminiscent of earlier collaborative text editors like etherpad, hackpad, or usecanvas. I think the hard part would be resisting t

At face value, the idea feels reminiscent of earlier collaborative text editors like etherpad, hackpad, or usecanvas. I think the hard part would be resisting the temptation of folding your markdown (hiding # or *) or moving to WYSIWYG. For starters, you can take inspiration from apps like bear notes, ia writer, ulysses, typora, or even apple notes (which has collaborative editing nowadays).