As I said in the Shanghai thread, I think his motivation dipped a bit after the US Open - that must be really demoralizing. Roger will remain an elite player as long as he can find that motivation. He doesn't have anything to prove, so to go out there week after week and train and perform at the level he needs to must be difficult.
As for "the decline," others have already said it but it should be re-emphasized. "The decline" started years ago. He absolute peak was 2006. 2007 was a fraction of a step down (compare win-loss %, although he did just as well at Slams). 2008 was marred by mono and Rafa's rise and even though he returned to #1 in 2009 he wasn't quite as good as he had been a couple years prior, plus Rafa, Novak, and Andy had all risen to elite level. All factors taken into account, I think "the decline" really became noticeable in 2010.
I think he'll prepare for 2016 like has any other year, but with even greater determination and a sense of urgency. I'm not sure how this will impact his play. Roger does best when he's come and cool, not when he's stressed and pressed. If he can maintain his Zen-like demeanor, he will remain a challenger at most Slams.
As of this writing, it would be hard for him to win the Australian Open or French Open - there are half a dozen or so players that are favored over him at either tournament. But if he's in similar form next summer, only Novak is the clear favorite over him at Wimbledon or the US Open, or the Olympics for that matter.
2016 is a huge year for both Roger and Rafa. If Rafa can't find his dominant clay form, and if Roger goes another Slamless season, I wouldn't be surprised to see both hang up their racquets.