I think Cena SHOULD turn heel. sure he makes a lot of money in merchandise, but do you know why? Look at him when he comes out, he's rocking his own hat, shirt, wristbands, armbands.... hell I'm sure he's wearing John Cena boxers, then he throws all of that stuff out to the crowd and comes out with new stuff the next week.
In all seriousness though (which means I was joking in that first paragraph, I'm sure there's some genius who didn't realize that until now) John Cena does sell a lot of merch, but nowhere near as much as Hogan did for WCW back in 1995, and his heel turn not only made them more money but it sparked a storyline that not only brought WCW to it's creative and financial peak, but caused the WWE to up their game and completely revolutionize the industry... although on the flipside of that Stone Cold turned heel back in 2002 and it was one of the worst decisions the company could make.
The point is sometimes you have to take chances and shake things up, break away from the status quo... and the status quo right now really sucks. Will Cena turning heel spark another "nWo" size storyline.... probably not, but it could, but at the same time I don't think it would be anywhere near the disaster of Austin turning. You can't be afraid to make risky decisions, that's where true success and prosperity ultimately lies. I say turn Cena heel see where it goes. Not like you can't undo it if it flops