Add “chapter” between “section” and “page”. Section Groups are useful, but do not fit the information hierarchy I am interested in creating; chapters will allow a better organizational flow of information.
1. I think this idea does not belong in the developer APIs / developer platform section as it requires an agreed change of product prior to any change to APIs.
2. What is your end goal? Are you trying to use OneNote for writing a book, or would a "chapters" feature be useful for personal note taking as well? Can you provide a specific example how chapters would be used, where section groups doesn't make sense?
Andy T commented
You can create an infinite tier of section groups. If you like you can refer to them as chapters...
I do this. I only have one notebook. Then under that I have Several Section Groups, which in them have section groups. and so on.
The UI for this isn't ideal, but the glacially slow pace of OneNote Development shows that there is only a single intern working on this project, so you are unlikely to see anything new anytime soon. Everyone I think was pushed to phone app development. Or maybe designing icons. As its about the only change Ive seen.