Drawin on non planr surface sound really good! Have no Idea how to make it precise, just do not know logic that you are going to implement.
Another Thing that is reaaly needed is the ability to move Cplane picker in ZXY coordinate! It will be very usefull when drawing on non planar surfaces!!! For example I alwas need to move picker a bit forward from sruface. This feature would be greate!