This. A delusional and egoistical guy turned into the "hero" by the machinery of the WWE. The commentators must talk good about him, he must be in all the posters, he receives unlimited title matches and his time is always billed as the most important. He is beaten 90% of the matches but then wins with the four moves of doom. He buries all the young stars that one day could overshadow or threaten his position, but always in a way that makes him look like the good guy despite all the crowd knowing is just hypocrisy and the company forcing us to love him. I even could see the referees cheating for him. He will continue to be the guy of the corny jokes and the colourful t-shirts.
Man, I miss Rock and Austin. Oh wait, you weren't talking about Rock and Austin? Oh, my bad.
Seriously though, kids love the guy, and he does nothing as a heel that no one else could do in his position. What you're talking about is a character that would have nothing to do with what made him popular to begin with, like turning Austin into a corporate hack in 2001, it was a complete failure. Rock was a great heel because he was funny (and also a corporate kiss-ass, pretty boy, so maybe similarities there), but he was also kindof a failure as a heel because ppl continued to cheer him (yeah, they did, except live audiences after he expressly pissed on them), and it was just a three month stint. I'm assuming Cena's heel turn would be more than three months.
The only successful heel turn of an extremely popular character was Hulk Hogan. So, if you think shit will get hot like the nWo got hot, then turn Cena heel. But, I don't think any faction or nWo-copy will ever shock and entertain like the original did just because of high expectations.
The desire to turn Cena heel is based on ppl who don't like his character altogether. Ppl who say he's become stale are mainly ppl who thought he was stale yrs ago. His legion of fans like him today just as much as they used to. And, I promise you, a lot of Cena haters will cheer for him for turning heel, it won't make sense.