I don't know what the new HMs will be, but if I have a feeling it'll take advantage of the DS features. Of course, though, Rock Smash did start out as a TM.
It would have to be something that would help overcome an obstacle that blocks your character's progress.
Maybe Howl will be one of the new HMs. All you would have to do (as silly as it sounds) is howl into the microphone and maybe it'll trigger something in the game.