Look, no doubt Cena is a huge draw and the face of the company. He's my favorite wrestler as well. It was a serious blow when he was injured back in 2007, and also a serious blow when he was injured in 2008. But no matter how valuable someone is to a promotion, they're replaceable. WWE alone this year has lost Batista, Shawn Michaels, and Chris Jericho to retirement/leave as well as HHH and The Undertaker to significant injuries. And they've managed to plow through without any of them. WHy? Because they have done a phenomenal job of elevating younger talent. Cena would be the bigggest blow however, so something would need to be done. In my mind, there are really one of two options here. Either bring back the big name, or go all the way with a push to the main event scene.
The big name you would bring back wouldn't be someone outside of the company, nor would it be someone whose on indefinite leave. It would be HHH. HHH was rushed into the main event scene upon his return from injury in 2007 because of John Cena, and theres no reason why he shouldnt be considered for such a role again. A face HHH is like Cena in a lot of ways in that he makes fun of everyone, and is known for his intense brutality. So in essence, HHH could pick up the feud with Nexus and CM Punk that John Cena started. His return "grudge feud" with Sheamus could hold for the time being.
The other option is the elevation of a young superstar, possibly moving one from Smackdown to Raw. My choice here would be Kofi Kingston. He was in the middle of the biggest push of his life when he was feuding with Randy Orton about this time last year, and WWE essentially dropped his push upon his move to Smackdown. It was a been there, done that scenario when it came to him winning the IC title. He's remained in that title's picture for so long now because they likely havent had anything for him, and thats fine. Kingston wouldn't be able to step in and fill John Cena's shoes as the face of the company, but he doesnt have to. He and Punk are former tag team partners the same as Miz and Morrison used to be, and Kofi has fallen victim to the Nexus number's game before. He's shown himself capable on the mic in the past, and against a big time player such as Randy Orton. Kofi could use his "knowledge" of CM Punk and their time together as feul for the feud.
I'm not saying either would be as successful as a Punk/Cena feud, but noone can truly fill the hole left by Cena. It will liekly take a myriad of guys stepping up as well as an early return for them to successfully get by without Cena. But one thing WWE has shown itself to be capable of is functioning without a top star for a significant period of time, and theres no reason to believe that this one will be different.