If they want to turn Cena, the time is now. I would let Rock win at WM setting up their rematch at the next PPV. Cena goes nuts after the loss, tears up the set, etc. On RAW the next night, he comes out, apologizes for his actions, tries to get sympathy, etc. He calls out the Rock to congratulate him and shake his hand. Cena shakes his hand, then jumps the Rock. He beats him, stomps him, hits him with the belt / chair, etc. Cena makes the complete heel turn. He wins the WWE Title at the PPV after WM. Rock leaves to film a movie.
Punk loses to Taker, wants to take time off and returns in a couple of months as a face. He talks about losing the belt, losing to Taker, taking time off to get some perspective and re-evaluate things. He's the guy that is the thorn in Cena's side, becomes the #1 face in the company and setting up a Punk v Cena WWE Title Match at Summer Slam.
Ryback spends a year getting built up as a top face in the company. This sets up a Ryback v Cena match for the WWE Title either after Summer Slam or at Mania (with Ryback winning the Royal Rumble).