My thought is that it wouldn't be that big of an adjustment for Roger to make since he's been dealing with that same tactic from Djokovic for the last 8-9 years. Just about all the talking heads suggest that tactic for anyone who plays Roger - play to his FH down the line. So I guess I just don't see the surprise element if Nadal starts going it, too, when everybody else is doing it. What's Nadal's next shot when the ball comes back? Cross court to the backhand regardless of if Fed's return shot goes down the line or cross court? I think Nadal would have to think 2-3 shots ahead if he makes this adjustment and hope that Roger stays with his usual patterns - which this year has been much less predictable for him. Roger's surprised him a lot by just stepping into the court in the middle of a rally and thumping the ball at a sharp angle cross court and then if it comes back thumping his next shot hard down the line in the opposite direction. At times you've been able to see Roger setting up his winner 3-4 shots before he hits it - that deuce point in The Rally of the Year at the AO is a perfect example of that. He just kept hitting the ball shorter and shorter and at an angle to Nadal's backhand until Nadal hit a backhand deep to his forehand side and then he flicked the kill shot forehand down the line on the opposite side of the court.
So yeah...Nadal can make the adjustment you're suggesting but he's also going to have to adjust his next shot if Roger gets the ball back in play. He can't just surprise him with the 1 tactical change. It would help him considerably if he was a better server because then he'd win more cheap points with aces and unreturnable serves. Roger dropped 20 places aces on Nadal in the AO final - 3 of them in the final game to climb out of 15-40 and clinch the title. How many of those aces just barely caught the line? One of the commentators in Shanghai was saying that most players who challenge Roger's aces are doing it because they just can't believe that someone who isn't blowing it by you at 130mph+ can so consistently hit a piece of the line. You saw that in Shanghai - how many times did Nadal challenge an ace and was wrong? At least 3 times. So that's my suggestion for Nadal - serve better and win some more free points and make your life easier. He prefers a higher percentage and getting into the rally. At nearly 32 - hard for old dogs to change habits but it might be time to make a change.
Completely disagree with most of this.
I'll start with the second paragraph, Nadal's serve was the least of his worries in Shanghai vs. Roger. He actually held serve quite comfortably for most of his service games. If you compare it to previous Fedal meetings on all surfaces, Federer used to be able to get into Nadal's service games much more often, but he had awful break point conversion ratio and most of the big points went Nadal's way. In Shanghai, Nadal was holding comfortably but all Roger needed was to get into one of his service games and he played the big points better, much more aggressively, and more relaxed than in the past, which makes all the difference.
As for the first paragraph, and I really don't want to sound like a jerk here, but it's mostly irrelevant. Novak's tactics against Roger don't matter because the lefty dynamic with Nadal changes the whole match-up, for better or worse. Novak will hit most of his forehands cross court to Roger's forehand because that's just how tennis rallies work (mostly cross court exchanges). This is the usual pattern of play, coupled of course with backhand to backhand exchanges in which Novak's threat of firing the backhand up the line is something Roger has to watch out for. This has nothing to do with the Nadal match-up.
Also, you bring up Djokovic's tactics with such dismissal that you'd think Roger has been whipping him all these years. He hasn't.
To be clear, I think Roger will continue to get the better of Nadal but somehow bringing up the Novak match-up coupled with dismissing how difficult it would be for players to deal with Nadal firing the inside out and DTL forehand early in the rallies is silly. Of course, for Nadal to actually do that, he'll have to deal with Roger so expertly taking time away from him and stepping up the court. THAT will be the challenge. But the notion that he'll have to think 2-3 shots ahead if he hits an inside out forehand is ridiculous. This isn't chess and that's not how tennis works.