Head tracking would definitely be one. Although rough position tracking with accelerometer rotation tracking might work just as well. Still these are much lower cost than all the hardware that went into the wiimote head tracker.
you'd think that this setup would be much lower cost, but from a quick lookup, these linear sensors (TSL202R) cost more than $8 each in 1k quantity, while you can get a wii controller for about $20. On the other hand, this setup allows for 1000 fps while the wii controller's camera works at 100 fps.