First off all, CENA WILL NOT TURN HEEL. Cena is too big a superstar. He's still the face and is selling great merchandise. Vince will make the biggest mistake of his life by turning Cena heel. So, this is what will happen:
He costs Barrett the match at SS. Yeah, you read it right. Cena will make Orton win. Next night, he makes an emotional statement that he can risk his career but he will not make it by wrong means. He's fired. No signs of him for the next 2 months. At the Royal Rumble, Barrett and Nexus are the last 4. Taker's gong hits. He's trying to eliminate everybody but can't. Finally, he get's the mic and says that he has another option and it is... JOHN CENA. Cena comes down and fights off Nexus with Taker. But still they can't win it. They eliminate the 3 members of Nexus and then brawl with each other. Barrett takes advantage and wins the thing.
For the next 2 months, they stalk each other. Cost each other the matches at E.C. Finally Taker comes out and says that he's tired of this. He says Cena was the reason he got Buried Alive and he wants Cena to profess it in front of this audience. Cena comes out and says he has nothing to do with it and asks Taker why does he feel that way. Taker says because he has never faced Taker at WM and wants to. Cena says he doesn't want to face Taker because he doesn't want to embarras Taker and also the others who have tried and is about to leave when Taker challenges him to a match. Cena accepts and says he's gonna end Taker's streak. The match is on, and Taker wins.