I may be wrong, but does Cena really have that much direction over his own character? Shouldn't it be more like "How does Vince Mcmahon (or whoever is doing the scripting right now)do it?" I had heard that Cena actually wants to go heel, but Vince will never allow it. I haven't done much research behind that statement, but whenever I see him come out on Raw each week Cena just seems to have this look on his face like he is just as tired of his unchanging character as everyone else who hates on him. As long as he is an icon for children, as Hulk Hogan was, I don't think things will change unless ratings plummet and something similar to Hogan turning heel needs to be done with Cena.
It's obvious that Vince is using Cena's character to help Linda, this hole rise above cancer thing is just a way help Linda. Cena's character sucks and I think he knows that, he also knows people wouldn't stop booing him regardless how much of his charity work they show us, the only way to stop the boos is by turning heel, but Vince wouldn't let it.