Antonio Vazquez (antonioya) is working on a set of tools for the Grease Pencil that will add features like animation interpolation and even ballistic effects. The code is still work in progress, but here's a sneak peek at what he's working on.
I'm working in some tools to improve the grease pencil for 2D animation.
All operations are controlled using the mouse and the viewport widgets use OpenGL.
This can be used for 2D animation or as reference to create 3D animation on top.