The notion of Cena turning heel has always been on my mind from time to time, but I would always resign my ideas when considering how much of a cash cow he is for the WWE as their super face. However, there have been several events as of late that have piqued my interest and made me take a renewed shine to the possibility of Cena actually becoming heel:
1) The recent upswing in CM Punk's popularity. If the reason behind the head offices refusal to turn Cena heel is the lost of their top face, not only has Punk already filled those shoes, but he's lined them with pipe bombs and exploded the company's revenue. Him becoming the company's new top face in the near future is almost inevitability.
2) As Rock Bottom mentioned in his original post, Cena's new attire is certainly a change from the norm. Minor as it may be (especially when considering that an explanation for it could simple be that the WWE had ran out of colors for him to wear) It just seems to give him an edge in his persona that I had not seen in awhile.
Now, as to the subject of his theme, I agree that a change needs to occur. Not only has he had his present theme for a god awful long time, but a new theme would give him a fresh flare and really bring his new look together. As to what that theme should be? My pick is this:
http://www.youtube.com/watch?v=a1hIphLhx_w
If I'm not mistaken, I believe Cena has already hinted that this would his new theme come wrestlemania, and I think it would fit a heel persona.
(Also, I apologize for not embedding. I'm honestly not sure how too...)