I doubt WWE would ever do it, but a heel flip for Cena at Mania would bring some excitement after the show. Im sure it would hurt shirt sales with kids, but Cena needs a change and the show really needs some new story lines imo. I miss quality heels in the WWE ala the Attitude era. I think Jericho is about the only solid heel they have left. Cody Rhodes and the Miz as heels is a sad joke. Johnny Ace has improved a lot, but he is still a poor substitute for evil Vince.
Cody is a great heel, they also did a decent build of the match where I go, hey, this guy might actually have a chance against the huge ass Giant with the fist punch from hell that shatters skulls.
Cena will not turn heel, heel Cena doesn't sell merchandise like face Cena does, it would destroy a nice portion of WWE-income, it won't happen. There's been heel's in the attitude era that sold merchandise and got cheered like crazy for it, you know who. But Cena is being legitimatly hated by half the crowd, if he turns heel everyone will be booing him out of the building and really hate his guts enough to NOT buy any Cena-merchandise. Yeah, it just won't happen.