Kinnect be HD? Not sure what you are asking when Kinnect games are HD. As for the camera itself, it is pointless going into HD. Kinnect already can track eye brows, so, IMO, it is already HD enough.
The Kinect camera uses only one fourth of it's resolution and it is a very poor resolution if you ever tried VideoKinect. The resolution is reduced to make sure that the USB bandwidth is not used up completely by Kinnect. By using USB3 it should be possible to have a HD camera and still have enough bandwidth for other devices. But a higher resolution also requires more processing power for the body tracking. So it is a trade-off.