Cena's the poster boy of the WWE. He won't be fired. His last firing when he let Orton win at Survivor Series against Barrett lasted all of about two weeks. Also, I can't see them turning him heel either due to the fact that he is possibly the greatest thing merchandise wise to ever happen to the WWE (Hogan, Austin and Rock included).
It's more what they can do with CM Punk and the title which is the interesting thing right now. That's what everyone wants to know. Everything else is secondary.