There will likely be at least 2-3 faces at the top of people who haven't even debuted on the roster yet, so it's hard to say.
Cena still has probably a good 5 years in him before his injuries start to catch up to him.
CM Punk will continue to be a top guy in some form for as long as he sticks around, in my opinion. Depending on how well his future goes, he may decide to stick around longer than he's promised to, just like he did when he was about to leave the company in 2011 before the MITB angle was proposed to him.
Nobody cares about Sheamus. He constantly loses an average of over 200,000 viewers in the ratings most weeks (on the 1/21 Raw, he lost over 600,000.) I don't think he's that bad, but they need to seriously do something with him.
Del Rio is impressing me with his face run, and if he can keep it up, he can be a top face (even worthy of holding the WWE Title, which is far more prestigious than the WHC) for the next several years. He's almost 36, but that's about the time when a lot of wrestlers hit their prime.
Ziggler does not impress me as a future main eventer. He is a great wrestler/worker in the ring, but he is mostly average on the mic. He's never struck me as someone who will carry the company for any lengthy period of time. Most likely will never go past the WHC, which isn't even the main event, unless he develops a better gimmick other than the "show off." It's also funny how some people harp on Sheamus's age being close to Cena's as a way of mocking WWE for trying to use him to replace Cena as top face, yet never bring up how Ziggler (who they think is a huge future main eventer) is only a couple of years younger (yeah, early 30's isn't old, but neither is Sheamus.)
No telling how good the Shield will or won't do until they break off onto their own paths.
I don't care enough about most of the other guys (Miz, Barrett, etc.) to even comment on them.