When you watch the trailer you see that the mask of light was hidden in a bolder, and begins to glow when it is released in the lava. What if Mata Nui's spirit was inprisond in the mask of light while he was set in his eternal sleep. Now with the mask being found his spirit can be released and make a seperate body (takunuva) to help the toa protect his island while he recovers from his long sleep.
So, what do you think? Could I be on to something here, or really far off?