A way to rig whitch helps with animation

My idea is adding A way to rig. I would like a way to rig so if you have A character with joints for example. That if you then move the upper leg the bottem leg wil rotate the same amount. But that you can also move the bottem leg on its own. How i would do it would be: i would make a new tool where you could rig with and if you than select the point of rotation it rotates around that point for what pixels are integraded in that part of the rig. This would be nice for animation in my opinion to make it faster.