If they do Rock/Cena III, then I'm seriously not watching that PPV. This is why I hope the title match this year does indeed turn into a triple threat with Punk, because then it technically couldn't be a one-on-one 'rubber match' between Cena and Rock next year. And what could they possibly add to the match? Last year's was the first ever encounter between them, this year is for the WWE Title, what could raise the stakes for next year? Maybe a title unification?
The good thing is that Cena has mentioned (in kayfabe terms) in an interview that his dream match for Wrestlemania 30 would be against the Undertaker and it shows there's probably already long term plans for the match to happen next year. And with Taker's career dwindling down and next year being the big 30 year anniversary, there's no better place to call it a career and no better opponent to go against than Cena.
I'm betting on Rock's match next year being with Brock instead of at Summerslam. A tad bit annoying since it shoots the WWE Title match down the card into the third spot, but it would be a huge double main event regardless. And I'm fairly certain WWE realizes that that double main event would draw bigger than Rock and Cena for the third time. Hell, Rock/Brock alone would draw bigger than Rock and Cena for the third time (mainstream Hollywood star versus a mainstream UFC draw.)