If this project interests you, check out GoodNotes.

I've been using GoodNotes for a year now on an iPad Pro with a Pencil and for me it's a killer app. Since I've started using it, my handwriting has improved. That's not something I set out to do, but it happened probably because I wanted to help out the OCR engine. It's pretty awesome to run a search and have your handwritten notes surface.

My worry is that the app only sells for $8 and that doesn't seem sustainable. The built in Notes app keeps getting better, so maybe the inevitable outcome is that the third party apps fade away as people switch to the core app.