Allow zooming and panning during painting
Zooming and panning while painting allows to block out objects quickly with only a few strokes (and some keyboard assistance to change pen color).
At the moment, blocking out abilities are extremely broad only, and you need to add smaller details as a separate action after reducing brush size and zooming in. Being able to zoom in during painting would allow greater precision to allow the medium-size details to also be blocked out quickly, as part of the same single action.
When zooming, it would need to zoom in to the current area in such a way that the pointer's position within the window still makes sense (ie. if you were painting on a particular pixel before zooming, you would still be directly over that pixel after zooming.)
MyPaint supports zooming/panning when painting, so I was able to make a mockup demonstrating what can be done in very little time. The attached image of a traffic cone was drawn in 3 strokes (== pen down, pen up pairs):
Stroke one: Define the silhouette
Stroke two: Define the lighting (using keyboard to change palette color, and brush size)
Stroke three: Disconnect the bright band on the cone from the front of its 'foot'
(image shows breakdown of strokes)