This has probably already been said but I was thinking a Horror/Sci Fi game with the hand held as an infa-red screen and the tv screen really dark to give a real atmosphere. You could even have invisible enemys on the tv screen and only seeable on the handheld. In adventure games the game pad could be like a scanner that picks up clues and hidden areas.
Sorry if this has already been posted and talked about but it is pretty exciting thinking of ideas developers could come up with.