add more useful features such as..
Useful features would be:
•ability to set passwords on notes and it will show no preview for them in the note page (could show a lock symbol)
•Ability to change eraser shape, mainly so it could be round, or like a brush stroke. It would make erasing much more accurate.
•more text formatting options
•different types of brushes for sketching, like some grunge ones for effects.
•LAYERS, along with layers you should add opacity for them, and maybe some effects like multiply, soft light, screen, etc
•free cloud storage, but its hosted by you, users only get like 1GB or 600MB. (This would allow users to feel their documents are safe, howevef eventually they need to buy the upgrade)
•some kind of gallery sharing option, users can upload their sketches to an in-app gallery, other users can comment on pictures, you can look through the gallery at other's images. Search for images, have genres, set your images to private or so only specific users can view them.
•Palm regection for galaxy note users.
That's all that came to mind at the moment, they could all come in one $9.99 pack, along with other suggestions you liked.