This is really cool! I'd love to see it taken further, and as others have mentioned have more reason behind the drawing. I think you could start simple (programming wise) by having the drawing contribute more to the generation of paint. Make the player have to mix colours on the canvas by painting over them to unlock new ones! That sort of thing.
I think there are tons of directions you could take this, very interested to see where it goes if you keep working on it!