Anyways, let me take a shot at the question asked by the OP.
There are a couple of things Nadal can do to be more competitive and it has more to do with his backhand than it does with his forehand. Naturally many will say he needs the forehand down the line, but I think even more important than that are the backhand deep cross court into Djokovic's forehand and the Nadal backhand DTL into Novak's backhand. When Nadal hits that backhand cross court, deep and with pace, he stops Novak from exploiting acute angles that pull Nadal further and further off the court. What normally happens when Nadal hits this shot is that Djokovic has to take a step, and he is forced back behind the baseline to hit his next forehand, which often gives a slightly weaker reply. The reason being is that Djokovic's weight is transferring onto his back foot, instead of it going forward, which is what he is trying to do, and this results in a shot that Nadal can now exploit.
Nadal at that point will have time to dance around his backhand, and bring his lethal inside out forehand into play and this shot is a killer for anyone, including Djokovic. Nadal is now in control of the rally and can either hit a flat ball down it's gradient and blast it past Djokovic, or he can impart heavy top spin which keeps Djokovic pegged into that corner, forcing him to scramble. Rafa then has the option to either keep bludgeoning Novak until he gets an error, or hit into the open court by moving himself towards the centre of his side. This is all set up with a deep penetrating cross court backhand, and something that often is overlooked.
Nadal's backhand DTL is the next crucial shot, and the reason for this is that Novak is naturally moving to his right, expecting the shot to his forehand, when it is a neutral rally. Rafa has often caught Novak in no mans land when he executes that backhand down the line, the length of the court is smaller, meaning Djokovic is not only readjusting his feet, but also has less time to do it than if the ball went cross court.
I don't need to talk too much about the forehand DTL, but one of the big reasons why it is harder for him to hit it on a hard court is, unlike clay, the ball skids through the court more, and also gives him less time to hit the shot. So the difficulty for him is higher to do so, because Djokovic actually attacks Rafa's forehand, looking for the short ball so he can redirect into the exposed backhand corner of Nadal. It is obvious why Nadal cannot pull that shot off as much on hard as he can on clay against him, which is why the backhand cross court becomes the real weapon which he can use far more regularly.
As for serving, he needs to tie Djokovic up more often. It doesn't always work, but he needs to keep Novak honest, and aiming more shots at the body will tie up Novak's limbs for weak replies.