I'd be surprised if it never happened again. I think all that's necessary for it to start happening is for TNA to actually start running a competent product. Which I do think they have the talent to do.
But yeah I think basically the key to turning him is to make a dominating heel force that people tune in to see. In fact I think that's the beauty of Cena and why it would be unique from the other dominating heel forces in that he wouldn't be a stable-- you can pretty much make him run the show alone because of his character. It's seriously like having Superman turn bad. Superman wouldn't need a band of underlings to rule the world in the comics if he went bad-- he could just dominate everything because he's freaking Superman.
And the point of building Cena is to get people to start watching the show again-- then every hero he crushes draws people in more, and every hero that gets crushed gets built in the process. Cena is a huge bad guy that everyone wants to see drop and so more people tune in. That's the key.
Also the Immortal storyline didn't accomplish anything because it wasn't good. If it felt like something that wasn't a desperate rehash of 90s and actually had a progression that made any sense at all, it could've worked fine!
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I tried to have a fierce rivalry with SuperNiceDog, but he made me look worse than a GameFAQs Contest!