I've found myself reading through the various books and wishing I had a way to make notes. In previous lives as a player I have made notes as I went. As a DM who uses D&D Beyond primarily I find myself wanting to make make notes ad links to other pages or even other books. I'd really like a way to add annotations with clarifications, hyperlinks, home rules, and simple notes.

If there were a way to see community notes as well this would be great. Up/dow votes would be essential, Reddit style visibility. Possibly a several classifications for visibility that readers can choose to opt-in. All notes with no regards for voting, highly regarded, new needs votes, and similar.

I'd love the ability to make my own notes. I don't think community notes would be helpful to me but I don't object to them being implemented. 

Community noted done well could be good. I'm thinking something like the way Amazon does it for the Kindle.

There is a feature request for this: https://dndbeyond.zendesk.com/hc/en-us/community/posts/360027817454-Ability-to-add-notes-to-published-campaigns

I suspect it needs some more votes to get the attention of the developers.