A feature I use a lot when drawing in Autodesk Sketchbook is the screen rotate gesture. That is, the ability to rotate the canvas when using the two finger zoom getsture. The canvas can be rotated to any angle but will easily snap back to the original angle and will return to the original angle when Zoom 100% is clicked.
I really miss this when drawing in FlipaClip. I know it's possible just to rotate the tablet, but really this interupts the drawing flow far too much to be practical, especially as I often draw using a laptop stand to make my tablet more like a traditional drawing board.
If you look at images of traditional animation disks, you'll see that the ability to easily rotate the drawing has always been important in hand drawn animation, so I think this feature would be perfect for Flipaclip.