Okay, we all know Cena is going to get rehired. Its just a matter of what PPV he beats Barrett at.
But I have an idea I think could really work for his "comeback":
~ Cena appears on Raw after being "rehired", and goes on about fighting for the WWE Title
~ Raw GM comes on, and says because Cena was rehired, he will be treated like a new superstar. He has to work his way up.
~ Cena being Cena easily agrees to this, and a match with still US Champion Daniel Bryan is set up, as a #1 contender match for the US Championship (a way to give the US Title a bit more credibility.)
~ The match goes on, but Daniel Bryan wins by making Cena tap out.
~ Next week, Cena wants a rematch and gets it. But again, the match ends with Cena tapping out. After the match, he's visibly angry.
~ Following the 2nd loss, Cena runs out during a Daniel match and beats down Daniel Bryan, putting him through the announce table. ("My diet pepsi!")
Congratulations, you have a Heel Cena (His angry over losing and his pride of being a former champion losing to this "less superstar" being the turnpoint) and a great push for Bryan in those matches and the following rivalry. You can even introduce Bryan's nasty mean streak in this rivalry.