The ability to derive a parameter out of a bone's rotation would allow for many possibilities.
My thought is that it would be a float parameter that you could set how far a rotation on an axis would have to be to bring the float from 0 to 1.
There is a sort of jank way of doing this already with empty physbones and planes colliders that I have experimented with but it is a huge jury rig and does not work super great all the time, not to mention it's probably not fantastic for performance.
Some things that this could be used for as examples:
  • Corrective body shapekeys, such as when you kick your leg up so that your thigh doesn't pinch up into your hip
  • Flex shapekeys that could bulge muscles in areas when they are bent.
  • Other fun creative things I'm sure!