I really like the foldable Headers feature. I would strongly suggest a global command to fold all headers (except those that contain other headers), such that the entire document is compacted, but all headers are still visible.
As a corollary to this feature, I would be elated with a automatic table of contents feature. I’m not sure about having it within the document itself, but perhaps as pop-up (on macOS) along with the top-right (i) icon or vertical … icon.
This all integrates with search. When searching for text or tags globally, I want to find all the notes with them, but most importantly I want to see as quickly as possible all the areas of the individual notes that contain the text or tag. I would love to see the list of found notes indicate how many instances were found in each note, and the ability to quickly go to them. I find having to search globally, and then click in the note and search again within the note to be ridiculous. I should just be able to scroll through (or Cmd-G or down-key) all the search results globalyl and see the terms found in context. I’m thinking Headers might the solution: in the middle pane notes list, when a search is performed, under the note entry, you can list the note’s headers that reference the sections that have that search text in them (the actual content would still be displayed in the third pane–the note content). Then tapping the down key goes from search result header to header (with the number of found instances indicated somewhere) or even search result to search result, and then when the note has no more search results, down key (or Cmd-G) moves to the next note and its first header with a search result.