I’ve been meaning to launch a productivity tool that hooks into Microsoft OneNote for some years now.
I got really far a few years back. I taught myself C# and made a good go of writing a windows desktop app which utilised the OneNote COM API to talk directly to notebooks and pull the specific information out that I needed.
I was also working on a mobile helper app using Ionic & Cordova that was 90% finished; it even had a website.
The desktop app was hard work. The documentation was minimal and nobody else seemed to be working on OneNote extensions so I was having to learn everything on my own, reverse engineering things as I went. I was also having to traverse the steep learning curve that is the C#/.NET/WPF/XAML/LINQ ecosystem.