While on the MyPaint site I checked out their links to other open source paint apps. The one that got my attention was a thing called Webchemy, you create shapes with it and it has mirror modes so you can block out interesting Rorschach-like designs quickly. Some artists are using this in a serious illustrative way to produce complex images, for me its more an idea generating device, as the controls don't feel at all fussy and you can just start creating shapes without giving it too much thought.
The app exports PNG on transparent background and also SVG (scaleable vector graphic), so you can take these starting ideas somewhere else and work them further.