Roger won't be as dominant, he'll have a slight let down, but for the most part the guy on the other side of the net will be saying "fuck my life." He'll have a great year, write some more history, break ground, inspire people with what is possible at that age.
Rafa has the ominous knee strapping, but I think he'll have a good year. Not as dominant, but good. The way he's been playing indoors it's going to be sickening how good he will be on clay.
Murray's body is getting heavy on him. He's slowing down. He's straining. He doesn't have the great serve to get out of trouble so it's going to be the hardest for him of the 4. As it should be - he belongs among them but he was never on their level.
Djokovic is the biggest wildcard. I always downplayed his "crisis" as people were calling it, but the more I read, his interviews, that guru dude, that homophobic shit on instagram...it's weird stuff. His game was always a grind. It was always mechanical. The machine had to break down at some point. So much of it is mental for him. He had to build a fortress in his mind to be able to play that style. It's hard to see him building that fortress back up the way it was. But he has always followed in Federer's footsteps, so maybe we'll see a different Djokovic. A more improvisational, free-flowing Djokovic. I think he'll be embraced by the crowds a lot more, because of his absence, and that may rejuvenate him.