I have thought of a few ideas that may make this program atad better..
Airbrush
The airbrush would be a useful add-on so artists who use this program, will be able to have the same quality arywork as users of photoshop.
Digital screentone
How it would work
1) Select the area you wish to screentone.
2) Click and drag to a point you think is suitable.
3) Press ok.
The further to the right you want, the closer it would be, and thus the darker it is.
Well so far those are my ideas I may update if I have anymore....