Perhaps in a future version of MyPaint, it might be cool to come up with a virtual paper or media. This would basically be a 8-bit grayscale image or channel that would represent a heightmap, of canvas or paper, or even gesso with cracks, or concrete for airbrush tagging, so that drawing tools would interact with it, while drawing.
Perhaps even a second channel or image could be used for an absorbancy map, while using liquid painting tools, such as brushes, air brushes, and markers.
Awsomebump could help create the Virtual Media.
Even 8 bits within a 1mm range could represent steps of about 0.003mm, which is finer than a fingerprint on glass.
I should think that the painting could be stored on one layer and the maps stored on another.
I should think, to pull this well large samples would work better than small ones.
(I could scan a few pieces of paper, and convert them to heightmaps. )
While things like this can be added post-processing wise, but it the artist interacts with it, the result might seem more organic.