While Cena won't be replaced as the top guy until his body breaks down or until the public tires of him.
This has been happening for half a decade now
But yeah, in the current roster Ryback is the obvious choice. He's the most over 'newcomer' by far right now, has a great look, and if pushed right he can be the top guy that Cena couldn't, if you know what I mean.
Orton is 32 and a great choice aswell, but he isn't born to be a Face and as a Heel he gets cheered way too much. He should be a 100% Tweener, but with his Heel persona/actions.
★ "Everything you do in life echoes throughout eternity." ★ ---
I'm pretty confident in saying that there's no one on the current roster that can completely replace John Cena. You could make do with a Ryback or Sheamus, but neither will provide the versatility or worldwide appeal that WWE has come to expect from their top guy. If anything, we'll see a transitional time where the Superstar is replaced by a Star, like going from Hogan to Hart or Austin/Rock to Triple H. A move that saw the company slip on both occasions. The worrying thing about that is, the WWE are already on a downswing. They can't really afford to replace John Cena with someone half as talented.
Least of all Dolph Ziggler, Cody Rhodes, Antonio Cesaro or Damien Sandow. I love all four, but lets be serious. Those guys aren't replacing John Cena as the face of the company.
Why focus on why guy though. Imagine them grooming 2 or 3 and and keeping them apart for a long time until they finally feud. Try to create a heel,face and tweener of the future so in time they can becone what Randy and Cena were and what Cena and Punk are currently.
But a face of the company needs to be able to get the crowd going on the mic and ryback sure doesnt look like some1 who can do that
He is more of a Paul Heyman guy^^^(as in needs some1 to talk for him)