WWE's booking committee are contemplating a scenario where CM Punk would drop the WWE Championship to John Cena at SummerSlam to set up a rematch between Cena and Dwayne "The Rock" Johnson at Royal Rumble.
Johnson announced at RAW 1,000 that he would face whoever the WWE Champion is at the January pay-per-view event. Johnson would defeat Cena at the Rumble and then defend the title against Punk at WrestleMania XXIX. There is that scenario and the other is that CM Punk would defeat John Cena at SummerSlam to retain his title and then go on to face The Rock at the Royal Rumble. Johnson would win the title there and then go on to face John Cena in a rematch from last year's WrestleMania main event. As of last word, WWE didn't want to do a Cena/Rock rematch at WrestleMania 29 so the first scenario looks more likely. Either way, The Rock, John Cena and CM Punk are the three men WWE wants involved in the angle.
Of course, plans can and likely will change but this was at least talked about during backstage meetings at RAW on Monday.