Why John Cena can't be a heel?

I personally, and most of the fans are tired of seeing Cena at the top as the face. His gimmick has never evolved and few arguements can prove that its not gonna change in near future of may be forever. Every superstar has had a face-heel period in the company, take examples of Triple H, Mick Foley, Undertaker, Hulk Hogan Shawn Michaels,etc But John Cena ‘The Face’ has been company’s priced possession for a long long time now, and no doubt he is doing a great job to be at the position where he is now. There are few reasons why John can’t be seen as heel, the most obvious reason being He’s the most attractive character to the kids and WWE‘s ‘Be a Star’ initiative is doing a wonderfull job because of Cena’s presence in that team. He has granted the most number of wishes recently crossing the 300 wishes mark! Now a man who’s the face of such a big social initiative can’t be seen as heel? Isn’t it? Second reason being his personal interest, Cena is known to be influential behind the scenes as well and more often than not WWE management backs John Cena. Cena himself doesn’t want to change his gimmick and become Heel. It can be called selfish but at the end of the day he’s company’s most salable product. Fans can only wait at expect that John keeps his personal interests aside and try out something that he has never done in his career. A good storyline and the perfect opponent can make it possible. May be a certain CM Punk coming back and taking Cena’s position as the face and the same causing Cena to seek revenge? Ah one can only expect and keep guessing but the bottomline is John Cena will be the face for the coming yrs too. Sad but fact.