I would let the user have 2 or three views each one with its own background image, also a front,left,top key shortcut, like blender's 1,3,7.
Also the posibility to pan the view.
and extreme shapes for making humanoid character, like reptilians or fantasy characters.
I was doing myself with a background image and i had to be switching between backgrounds and moving the character to match the image. these features would speed up the process.