I couldn't see an exact match for this sort of request so here goes:

As a DM, I'd like to be able to add public and private notes to "my copy" of my sourcebooks.  I'm running Dragon of Icespire Peak for some true newbies, and I'd like to be able to tone down some of the monsters and encounters, add additional notes for changes to the script, and tweak bits and pieces. I'm intending to just have a ton of notepad pieces of paper, but unlike having the physical sourcbooks, I can't sticky the notes to pages in the online copies of the books.  

I'd also like to be able to add notes for the players to read, in the sourcebooks I've shared.

So the request is:

1. Notes would be text snippets, sort of like Google Doc comments, stored in the cloud and accessible on any device.  It should include markdown capabilities for dice rolling and tooltips, and linkability to other parts of the sourcebook (extra credit: link to other sourcebooks).
2. Ability to highlight a piece of text, and add a private note to it.  The note would be visible only to me, but accessible regardless of device when I viewed that book.  
3. Ability to highlight a piece of text and add campaign notes to it. When creating the note, you would select either "Public" or a specific campaign to share with (must have content management turned on).  The note would be visible to anyone that is part of that campaign (or anyone I'm enabling content management for if Public).

This would enhance our ability to use the D&D Beyond library as closer to having physical copies of the books that we can add notes to.