Nah, Cena turning heel would be a shock if it happened now if for no other reason than the WWE has been so firmly against doing it for years now. Half the crowd should not be booing you're face of the company, it's not good business. They had the perfect opportunity to do it to last week on RAW when he named Daniel Bryan as his opponent for SummerSlam. Bryan is so insanely over right now it's ridiculous and once Bryan was done celebrating in the corner, the moment he turned around Cena should've nailed him with the belt and made the turn. A major opportunity lost right there.
Also, what happened to when two or three guys used to swap a championship back and forth for like six months or a year? That really made the rivalries more intense back then. I don't mind guys having long title runs to establish the belt and the person as a legit champ. But a feud where a championship is flipped back and forth a few times for a bit can be interesting and something they should think about doing again especially if they want a rivalry to have some excitement to it.