Back in "the good ole days" heels were heels and a baby face was a baby face. Over the past 10 years or more that line has been blurred. I will make the argument that the last wrestler (not including Vince) to get good heat and be a good heel was Slaughter when he was doing the whole Iraq gimmick around Wrestemania 7. Say what you want about him, "he didn't deserve the title, he was the shits blah blah" he got great heat. He was doing the right gimmick at the right time. Heel Bret Hart was good and so was Hogans heel turn but, Cena could be better that all three of those.
What should have happened is a few months ago when the Shield first came in and was kicking the shit out of people it should have been revealed that Cena was the leader. But they missed the boat with that.
Hes already getting booed by every male over the age of 14. What they should have him do is go out one night on Raw and tell the fans he's tired of trying to be the "Mr. Positive, its okay that you boo me, I respect that, good guy" take off his stupid wrist bands, shirt and hat and say I'm not doing this shit anymore. I've been working hard to entertain you stupid people and you boo me and cheer for guys that look like trash like CM Punk and Daniel Bryan. Then tell all the little kids to shove it. Have the announcers apologize and say its not part of the show let them cut his mic and have Vince and Stephanie come out in total kayfabe mode and try to get him to stop . As long as he calls the fans stupid it will work great.