Expose the complete Object Model of Onenote
The OneNote API should be the API that the Microsoft team that develop OneNote themselves uses.
There should not be anything that Microsoft can do that a 3:ed party could not do as well.
There should be a set of basic classes/objects that remain fairly stable over time and then just about everything else should be addons.
Check out how Visual Studio is done, and then do something similar with OneNote.
At the very foundation OneNote is nothing but a distributed and replicated Database, so creating new data models/structures should be a trivial thing.
There could be requirements that there should always be a read only text representation available, so something can be shown on any platform.
Then all what needs to be done is to implement a plugin for each platform to edit and fully display the new data model.
Make OneNote a a real serious development framework and you have no clue what amazing things developers will do with it....you will see things you can't even imagine...
Mark Herman commented
I know what developers will do with it. I have had a class on my network for at least 2 YEARS. They can get through the net work by using falsified credentials and change your whole computer and take your pictures. All your data put it anywhere they want and there is nothing you can do about it. A work group I never join has a class on my network and has made my life **** for the past 2 years. They store their programs and notes in my one note and block me out. 3 thousand dollars in train Microsoft help has done nothing but irritate them. Three new computers trashed by there deprogramming them. All I want now is names and addresses of these people that have so much fun at my business and our lives expense. I have had to take classes and learn as much as I can just to try to get rid of them. When there is no punishment for these people because it is a class is bull. So there should be all kinds of restrictions and punishments because people do not care or have any moral's now days. They are just arrogant.