As I said before 2015 would be extremely important year for the Big 3. We can not say the conclusion for Nadal and Federer yet because the last slam of the year is still not played, but 2015 is definitely not a bad year for Djokovic.
But thing is not very good either. He turns 28 years old this year, 29 next year, and next year will a bit harder for him to dominate. He still can do it, but how well he will do at slams? That's where his tennis life will be judged by history.
He needs to win AT LEAST 2 more slams next year to keep the hope for getting in tier 1, the tier of elites. If he win just 1 next year, he will have maximum 11 slams going to 30 years old. And from that points there will be no guarantee that he can win slam, even just 1 more, at all.