Document fragments for comments are really interesting to me. 
Hopefully adequately nontechnical: 
* outliners can link to a particular chunk, but file-based notes rarely do. 
* I'd love to have [Chrome-type section highlights](https://www.theverge.com/2020/6/4/21280115/google-search-engine-yellow-highlight-featured-snippet-anchor-text) done more semantically. 
* I dream of [Quotebacks](https://quotebacks.net/) being able to be more of a live-embed than markup for a piece of text you copied out on your own. (e: original author being able to correct a typo and have it show up where it's embedded?)