Federer is probably the toughest player to predict. At some point, he will have to hit a significant decline (he will turn 32 next year), but it's impossible to predict when. With significant I mean ceasing to be a permanent top 5 player and Slam SF feature - he will never cease to be a top 10 player as long as he's healthy even after 35+ yo.
In 2013, he could either win another Slam and remain in contention for #1 or not reach a single Slam final and struggle to stay in the top 3/4. It's just impossible to predict: on the one hand, he'll become 32; on the other hand, he's Federer.... I certainly wouldn't rule out another Slam, especially if Nadal doesn't come back to his best.
Much as Federer's fans might wish to say otherwise, there's no denying that from 2009 onwards Federer has benefited siginificantly in majors from Nadal's questionable fitness.
He has shown he can still win majors this year, but one would suspect having to get through a field that includes both Djokovic and Nadal near the peak of their powers...well, that might be asking too much of a player over 30 years old, no matter how great he is.