There are not many traditional animation software that let you do bitmap animation and I think this is one of the strength of Pencil. However the current brush engine in Pencil is quite bad in my opinion. I have never managed to get a nice looking stroke with the pencil tool, even on my cintiq tablet.
I think there is no need to re-invent the wheel when there are many existing free libraries.
It is why I plan to learn a bit more about libmypaint (the library doesn’t seam too complicated to implement) and see if I can implement it in Pencil.
MyPaint brush engine is very good and it would make Pencil the perfect tool to make sketched and rough animations.
The lib is here.