Don't need a mouse... I need a trackball.
One like the M$ Optical Trackball, but built-in to the thumbstick positions. Thumbstick buttons would be replaced by pressure on the trackball 'socket'.
That would be the ULTIMATE controller.
To fit the existing controller specs, the X/Y values would be based on acceleration/velocity, once passing a certain threshhold.
Mice SUCK when you are trying to use it from your couch!!