Even though Rafa is in decline, he is still #8 in rankings and #7 in race. So, the feeling that he sucks here are caused partly due to expectations of fans on him. I think he can work his way into top 4 or better in time for the clay season of 2016 and try to have a good run there (i.e, half of the marbles out there on clay).
Leaving aside rankings and results, play wise, there are definitely two things he can improve.
1. His serve currently is abysmal. He can surely improve it. After all, serve is one shot over which a player has complete control. 2. He is hitting a lot of balls short and that can also be improved with more practice and confidence.
However, he is not going to get his movement bac to where it was in 2013 or before and so even when he "gets it back", he will be out of position many times. As a result, he will not be able to string together a series of tournament wins like before (recall that before, his pattern was to go on a winning spree once on). It will be alternating results of good run and then an unexpected loss. As an example, he might win MC, lose Barcelona in SF, Madrid in 3rd round, win Rome, Lose at SF in RG.
While I would not be surprised if he does not win another Slam, I am thinking he will probably win one more RG (most likely in 2016 as it generally becomes more difficult to win, the longer you are off).
While Roddick's position is that "he will probably never win a slam", my position is that "he will probably win one more slam".