I really enjoyed this but there are some features you could add to make it a whole lot more user friendly.
1) The ability to copy what you've just drawn, edit, move, and turn it.
2) The ability to copy and/or cut out a single object by drawing a box around it (or by tracing around it) so that you can paste it elsewhere, even in a different frame.
3) Adding on to the above ideas, perhaps an 'object invintory' where you could draw an object and save it so you can use it as many times as you want.
4) An eraser (having to hit the clear button everytime you made a mistake was a little annoying).
Most of this would probably be very complicated almost to the point of being impracticle but I hope I've given you a few ideas :)