Depends on how it's done but what if Cena never turns heel?
I think it's probable Cena never goes heel. In fact, I almost hope he doesn't because it'd be neat for him to look back and say that no matter how many booed him and hated him, he never gave in and succumbed to the hate and always kept his head up high. Would help make him an even more unique individual than he already is considering almost every other top face turned heel at some point in their careers, whether it was met with success or not (Hogan, Austin, Rock, Andre, Bret, Michaels, Goldberg, Batista, etc.) Imagine him mentioning that in his speech at his HOF induction. Also, a lot of the resentment for him comes from a dislike of his white vanilla baby face character and the fact that people can't stand that so many love him. If they turn him, there might not be the same kind of resentment and the people who boo him now will just cheer him later on, which'll just be the same kind of situation now, only with the people who cheer and boo him now playing reverse roles.
IF they turn him, it needs to be different than any heel turn done before. Different from Andre's. Different from Bret's. Different from Hogan's. Different from Austin's. This is why I dislike the ideas (though I came up with a few of them myself admittedly) about Cena attacking The Rock at 28 or 29 and turning heel. It seems too similar to Austin's turn, especially this year with Cena's 'need' to be WWE Champion again in the center of it as a direct similarity. It needs to feel original so it doesn't just feel like a copycat. I guess a slight mix of Hogan and Bret's heel turn while still being different will work, which is milking Cena's baby face run for as long as the kiddies cheer him, and once they mature into teenagers and start booing him out of finding him corny or played out, have him turn then, hurt by the same fans he never betrayed until they betrayed him.
I can't think of any reason why he would turn on his fans unless they turned on him first. After enduring all the boos throughout the years and still accomplishing all that he's done under that kind of pressure, him transforming his personality into something 'darker' because of feeling betrayed by the fans who always stuck by him when a lot of others didn't is really the only path they can take.