Stripping trailing whitespace.
Allowing Z and Y in addition to Ctrl+Z and Ctrl+Y.

Single-key shortcuts are better/easier/faster, specially if the other
hand is holding a stylus or a mouse.
Adding A, S, D, F shortcuts for changing the brush size.
(experimental) Changing number key behavior.

Pressing a single key will select a color. However, if that color has
already been selected, it will select the next color.

Still missing: updating the numbers in the interface.