Valve took out a patent a while back on a gamepad where the fixed D-pads were replaced with sockets that could accept a D-pad, a trackball, extra buttons, etc: http://gamerant.com/valve-console-controller-hold-137072/

I will be so Goddamn happy if they're actually making that thing. I love modularity, and a trackball gamepad would flawlessly combine the convenience of a gamepad with the precision of a mouse. Several groups have tried to bring one to market, and studies have shown their superiority, but apparently they were too weird to get enough preorders. Hope hope hope...