Congratulations on shipping your app! I like the design and the concept of avoiding forced hierarchy like folders etc. I think this concept is worth building on even more. For example do users really need a rigid hierarchy like post-it vs A4? What would happen if I would be able to click anywhere on the canvas and immediately write down the thought (just like in MS OneNote) and format this text after writing it down - give it a distinct background to make it that physical note feel? Personally I'd be happy as it would give more freedom. Now that experience is a bit broken - I can click anywhere on the canvas to create a post-it, but inside of it I need to click on the top row to even start writing. I hope this quick opinion will be useful/inspiring for you.