Hello everyone, I'm an artist and new Quill user, One thing I've noticed that it's usually hard to put stroke in right position. It could look nice from the one view angle and completely wrong and crooked from another one because It's hard to feel the distance from the viewers angle.
Adding atmospheric perspective or some kind of customizable fog probably will help to control depth aspect while painting.
One more thing I've noticed that sometimes it's hard to locate cursor when you try to buildup shape with the single color. And it's hard to make it accurately without gaps, because strokes on background and foreground have the same value and color. (For example, if you'll try to paint spherical shape you'll have hard times filling out all the gaps. ) Aerial perspective could solve this issue too because with it you will clearly see the difference between background and foreground.
I think that 3d grid could be good addition too (not only grid on the ground plane but vertical lines too)
something like this - http://www.fubiz.net/wp-content/uploads/2014/07/String-Prototype-Tube-by-Numen5.jpg
It's just my personal observations.