Put a microcontroller with a USB host port between a game controller and the computer, pass everything through, then remap.

Take Gametraks apart and remount their tether sensors in a row, to capture a koto player's pressing and pulling of the strings.

Rebuild a Gametrak into an Ondes Martenot-style ribbon controller: a finger ring on a loop of cord sets the pitch, and pressing the board sets the volume.

Swap the Gametrak's own circuit board for your own microcontroller, wired or wireless.