What would you have in mind?
I would strongly suggest a more enterprising brand of tennis, in which he's going for his down the line and inside out forehands far more often. It's amazing to watch Nadal go for SETS without going for his down the line forehand. I'm not talking about the kind of weak down the line forehand that he hits with a shit ton of side spin that ends up landing towards the center of the baseline. I mean the kind of down the line forehand he tried to go for early in the tie-break and narrowly missed.
He's not the same athletic specimen he used to, is a step slower, and is more prone to missing, so he just can't rely on his cross court forehand doing all the work anymore. It's not just going to grind players down with the same effectiveness on a week to week basis.
This isn't one of those generic "Nadal needs to be more aggressive and end points earlier" advice. I'm talking about one specific shot that he needs to hit often and accept that sometimes he's going to miss it, until he finds it. It's so frequent nowadays where he starts matches slow, especially in slams, and isn't going for that shot at all, then his forehand improves as the match progresses because it almost ends up being a practice session, in that when you play for 4 hours, you're going to find your timing to an extent. You could see this happening with Rafa in this match, but when push comes to shove, you won't have the confidence to make the shots you're not comfortable with at crucial moments.
Case in point, the early forehand miss in the tie-break, the awful forehand miss that would have given him match point, and the easy inside out forehand he missed against Del Potro at the Olympics on match-point. You can't just put a shot on standby for such an extended period of time and only go for it when you're feeling good about your game when you're never winning enough to feel good about your game to begin with. Nadal's always done that, except he was winning often enough to where sooner rather than later he'd find his confidence to start going for his shots.
He's went two years with one of his better shots looking like garbage simply because he doesn't have enough confidence to pull the trigger.
I don't want anything drastic. Not changing coaches and getting rid of Toni (the way I'm sure a person or two on certain forums are suggesting), nor tinkering with the serve, nor trying to play a less taxing brand of tennis. Nadal is who he is at this point. I just want him to be who he is when he's feeling good about his game, even when he isn't, if that makes any sense.