A John Cena heel turn is constantly talked about, though I've honestly never seen a good reason for starting one.
I've already rambled on the many reasons for why I think that it would make absolutely no sense from a financial standpoint, so I'll just make with the idea already.
John has a really bad year in 2015. John is challenging the current WWE World Heavyweight Champion at Wrestlemania XXXI and he sees this moment as a new beginning for himself and the Cenation. Just cuts promos saying that he isn't abandoning his fans, but that he needs to take his game to the next level to compete with all of these bright new superstars.
During the match a young man decked out in Cenation gear jumps the barricade and attacks John's opponent. John admonishes the fan and demands that they leave right now, the fan screams "I hate you!" or something which visibly spooks John. Later John gives up a semi-clean win to his opponent and congratulates his opponent after the match.
The next night on Raw John asks if he can please speak to the young man backstage. John explains in a calm tone that "don't try this at home" isn't just a slogan and that the rules for fans are put in place for their own safety. The fan gives John a dirty look and slowly removes all of his Cena gear before saying something like "All you see are dollar signs John. Just dollar signs" before storming off. John gives a cold stare as the show cuts to commercial.
The next week on Raw, John announces that he's going to do something special. John is going to give away free John Cena merchandise, and the crowd goes wild. HHH's music hits and HHH storms to the ring. John explains that he doesn't want his fans to just look like a bunch of dollar signs and that he really wants to help spread the joy of the Cenation. HHH explains that it's petty of John to just give away free merchandise when it doesn't truly affect John's bottom line, and that if John really wants to impress his fans he should just go without his salary for a month. John agrees, and reluctantly has to tell the fans that it seems that he can't offer his merchandise for free after all as that's decided by the marketing branch and he's very sorry for misleading anyone.
The next week on Raw, John is scheduled to compete in a rematch for the WWE World Heavyweight Championship. John doesn't show up, prompting HHH to come to the ring and explain that John hadn't invested his earnings properly and due to forgoing his month's salary he couldn't afford transportation to the arena. John leaves a kayfabe Youtube video or Tout explaining that he's very sorry, that it won't happen again.
The next week on Raw John is scheduled to compete in a six man tag team match, but is once again late to the Arena. The match plays out and John's team loses due to having to fight in a 3 on 2 handicapped match. John arrives ten seconds after the match was over, only to be attacked by his angry team-mates.
The next week on Raw, John pleads to HHH to reinstate his salary. HHH comes to the ring and show sarcastic interest in Cena's request. HHH throws out a kayfabe number like $10 million a year that John had supposedly been making while his co-workers had been making so much less and that even HHH barely makes that much. HHH explains that things have been so much better for the WWE since they've been saving money by having John work for free, but he'll reinstate John's salary if he defeats HHH at Extreme Rules in Hell in a Cell. John accepts.
At Extreme Rules, John comes to the ring without his usual entrance as that would be considered part of his salary reinstatement. The crowd shows less jubilation due to John's entrance being that of him simply walking to the ring. John grabs a mic and insults the crowd for demanding that he play a character while not appreciating his talents in the ring, John then attacks HHH with the microphone cutting his forehead open. John's match with HHH shows a more brutal side to John and makes the fans wonder if his motivation is the pride of his fame or the greed of his salary. John eventually wins after a blood bath involving nearly every weapon available.
The next night on Raw, John's music hits before being abruptly cut off. John comes out with a mic saying "Oh?! You don't like that?! Well how about THIS?!" and his new music hits playing a dis track to the WWE fanbase. Stephanie comes to the ring and explains that while she's had problems with the WWE fans before, John shouldn't mock them for buying a seat as that's crazy. John retorts that he was crazy to ever think that his job was ever about the fans, that everyone works for one thing and that's money.
John becomes the next incarnation of The Million Dollar Man. Yeah, that's the best I could do. I still say that making John a heel is a horrible idea.