It's gonna be John Cena.
Why? One. Simple. Reason.
GIVEN...that The Rock wins the title, it HAS to be Cena. The Rumble winner will get to choose who they face at Wrestlemania. We aren't seeing anything other than Rock/Cena, which I have been trying to avoid accepting for some time now, but all signs point otherwise. If a face wins, they will look like a huge chickenshit and an idiot if they don't choose to face The Rock. Hence, whoever wins the Rumble will face The Rock(assuming he wins the elimination chamber). WWE hasn't given out multiple rumble wins in the past decade and a half, but this year will be different.
I guarantee it. Hence, the spoiler. Red rep me if I'm wrong.