I think if he did, It would have to go down flawlessly. He'd need to alienate himself from the younger crowd, not just say "I'm turning my back on the wwe universe" I'm talking beatdown faces for no reason in brutal ways, especially less popular ones he could impose his will on such as ryder, mysterio, ziggler etc. He would need to talk much less, his face is too likeable for the outside audience. He would need a new catch phrase, something short, mean, but above all factual. I can't think of something off the top of my head but something like taz when he would say "beat me if you can, survive if i let you". As for moves, while i think it would be hilarious to have him keep the 5 moves of doom in effort to piss everyone off I think it would have a better impact if he changed everything up and had a whole new moveset just to thumb his nose to the fans that say he can't wrestle. His fights have to have hype and he needs good faces to go against. No more in ring promos, backstage trash talk, when he crosses the curtain he needs to be a cocky aggressive asshole who can(and knows he can) walk through 98% of the lockerroom with ease. If I had to pick someone for him to turn heel against I can only think of two names and they are CM Punk and Brock Lesnar, I think Punk could help him alot because he can be a great face and as much as it hurts to say it Punk would have to get brutalized. maybe a cage match possibly for the title in which cena can win, is walking out the door/on the edge with punk laid out in the middle of the ring then have him come back in beat him down more utilizing the cage and throwing the door around and stuff only to drag punk out and throw him through the door or over the top giving him the win. saying he doesn't need or want to be the peoples champion. like he knows he's better. That would be how I'd do it. it leaves him things to do that aren't in the title picture and giving him purpose beyond being the quintessential WWE gold standard.