Thanks for the suggestion. The app already has export and import functions, so it would be nice to have established notes. But I also think people need a note-taking and organizing something by themselves for learning. I think that's why people take notes even though they have well-organized tons of books.

I'm not really sure what it means for the forking mechanism. I would be grateful if you could show me a link to the resource for the forking mechanism you mentioned.

As for the tree structure, the tree structure is already reflected to some extent. As with breadth-first search, the questions starts from the higher level, but when the parent node is in the wrong state, the child node does not appear in the problem. I'll search more about the half-life regression you mentioned!

People would publish flashcard sets (share part of their notes tree) and others could fork and modify those sets, like GitHub. This is in my plans if I ever have enough time to compete in this space.

I think if you pay not much money for this, you'll get bad results. It's very easy to make memorisation materials, but it's hard to make good memorisation materials! Your best bet might be to convince a few Cambridge students that their path to victory lies in making these resources for themselves, and then pay them to give the resources to you afterwards.