This is unrelated to viewport rotation, but I've always wanted to play with multiple inputs for posing characters. There was always a lot of interest in "full body IK" which seems like a similar idea to me. Both IK and FK imply there's an anchor point and your single (mouse) input is posing the model. With two inputs it would be more like posing a puppet where one hand anchors and the other moves. You would still have to figure out how to deal with interpolation between poses, but I would imagine posing would be more intuitive.