The only thing that could douse the flames of my hate for Cena just a bit is if he beats The Rock by cheating. Chair shot while a ref is down, feet on the ropes, holding the tights, something....
BUT
...he doesn't actually turn heel. He celebrates just like normal, and comes out on Raw the next night and delivers his typical promo. Then with his first TV match post-WM he starts to change his wrestling style a bit. He would still do his whole Cena shtick, but add a few heel tactics to it. Holding until the 4 count, winning by count outs, pulling hair, holding tights, cheating in general, etc. Anytime people would bring it up, he'd just play it off and cut a typical vanilla Cena promo. Babyface in promos, but a weasel in the ring. Basically he'd be playing a phony douchebag because he realized that cheating to win was WAY easier than fighting clean, and without that change he wouldn't have won THE BIG ONE. All of this would build for months until someone like Punk called out his hypocrisy with a scathing promo causing Cena to snap and come clean on why he's turned into a complete toolbox.
From there every Cena feud would be relatively fresh, and crowd reactions would be all over the place. Hell you could even do Rock/Cena III with an entirely new spin on it, or Cena/Punk again with both guys in completely different roles. And if Orton turned heel again, you could maybe get Cena/Orton one more time with both guys trying to out-heel and hurt each other, but Cena's putting all the blame on Orton.
Troll Cena. Troll Cena is the ONLY way to redeem the guy at this point.