I think this year may be his best shot at a French title. Even if Nadal comes back, it's hard for me to think that he'll be playing at a level close to his best, even on clay. And even if he does come back, does he still have that aura of invincibility on clay? Or will players find the confidence to beat him knowing that he, possibly, may not be at his best?
He has a good shot at a Wimbledon title as well, though, I think Murray can beat him there and Federer will always have a great shot there as well. I can see players like del Potro and Tomic troubling him there, too. It's not like he's never struggled against them before and they are constantly improving.
I don't really think the U.S. Open needs much explanation. I think he can safely make the final there with little opposition, unless he meets an on-fire Federer or Murray.
His biggest obstacles are Federer and Murray, and possibly Nadal. I can't gauge how Nadal will come back and how much of a factor he'll be on grass and hard courts. A lot depends on how future draws pan out and, especially, where Federer and Murray land if their rankings exchange. I badly want to see Nadal come back to playing a high level of tennis because, if he does, we could be in for another awesome year of tennis!