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.