I don't know that you can do a real heel turn with Cena, at least not for a few years. The most you could do would be to set up a situation where there is a seriously split crowd reaction, with a strong competitor to Cena who gives the crowd reasons to cheer him and hate Cena. This would be a "we're beyond heels and faces" philosophy, so the anti-Cena would have to avoid clear heel actions or he's the heel and Cena's the babyface and we just have some Cena haters cheering again. The anti-Cena would probably focus on Cena's good looks and WWE Golden Boy status, so you'd want someone with a brutal history and not a pretty-face type.
Maybe Mr. McMahon aligns with Cena, and Triple H is the anti-Cena? McMahon pulls strings for his moneymaking Golden Boy--Cena doesn't cross the line into cheating, but he can't help benefiting from McMahon's help.
Maybe Mr. McMahon aligns with Cena, and Triple H is the anti-Cena? McMahon pulls strings for his moneymaking Golden Boy--Cena doesn't cross the line into cheating, but he can't help benefiting from McMahon's help.