A unique ring can show the other types of panner. I can imagine the shape of square of the panner we are using now, turned to a shape of circle in the ring of the knob.
Having 2 panner rings is a good alternative in the case of combined stereo panning. More than 2 would be a mess, but 2 rings could be a good option.
Another option is to have a unque ring with a label/marker indicating special panning and when cursor is on it show a floting square with the panorama.