The basic solution is to support common USB game controllers. A sugarized config program to remap game controller buttons would help.
The system could cheaply host an analog mushroom pad, similar to modern game systems. Such a pad would have a patterned ball backside sitting above the camera. The mushroom's ball bottomed stem would click into a socket that wraps from the camera down to hold into a usb socket. The usb connection would be about enough to physically secure it, and grab a little power to LED illuminate the control pad's backside image. Of course, some terribly clever software driver would be needed to interpret the image without using up all the CPU.