I don't think they missed the boat with Cena, but this break that he is about to get in making the movie should be more than a month or so. Make his comeback big. I'd say turn him heel and have him go over someone like Kurt or Taker at WrestleMania for the World Title. If it's against Kurt, he'd obviously continue being face.
They can set it up with Kurt and his gang beating up on Eddie or Show, and have him make the comeback and save. It would be interesting to see someone like CCC in Kurt's stable to make it more logical for Cena to attack them. They'd have to some how set it up in the upcoming weeks or so with maybe Luther or Jindrak and Kurt attacking Cena and taking him out instead.
Although him turning heel would be the best thing. He'll be able to sell more, make his opponents look better, and his spots would get a lot more heat. The only person he could turn heel against would be Undertaker, and him ending his streak at Mania for the World Title would establish him perfectly.
Cena really has the tools to be a Champ, IMO. He just isn't the same. He really isn't angry at everyone anymore, and doesn't walk around like he's all alone. When playing to the crowd he loses tons of edge, especially since he doesn't need to do that at all. A heel turn would be the best thing to do because then he'd go back to focusing more on his opponent than the crowd. Attacking someone like Michael Cole instead of saying he's gay would also help. Even attacking Tazz before he would fully turn heel.
But if he were at RAW, facing people like Benoit, Benjamin, HHH, HBK, and Jericho would only help. Turning heel there and making him the 2nd heel to HHH would also help. Even facing HHH too. He'd obviously have to not be buried for that to be successful though.