How would you execute a John Cena heel turn?

A Cena heel turn is a constantly talked about topic but, I believe that some people just don't understand how hard it would really be to turn Cena heel, I personally can not come up with a good idea that would turn Cena into a legitimate, believable heel character that he could be for a solid year or so and challenge the best babyfaces in the business. The reason I believe it would be so hard to turn John Cena heel and why WWE does not want to, is that his entire character that has been built over the past 10 years would be torn down. To be a strong heel, Cena really can't personify Hustle, Loyalty, and Respect anymore because it would go against the basic rules of being a heel, and then he wouldn't be a sustainable character. So, my challenge here is, which will be easy for some of you is to construct a Cena heel turn, where he has to turn heel by say night of champions this year, turn him into a sustainable heel character and walkthrough his major feuds during the run as a heel.