Unlike against any other team, Camp Nou is not a sure ground vs Real Madrid for Barcelona. That alone warrants a consideration of why this does not have to be a breeze based on it being a home game for you lot.
If we are to simplify this, three (or four) key things need to happen for Real Madrid to win this game:
1) Real have been offensively dependent on the success of the Fede-Kroos partnership. I am not even considering Modrić start, because we simply cannot afford it, he can come on for Kroos or one of the wingers to provide more experience from 60 mins onward, but his lack of pace cannot be hidden anymore. Fede is hands down our fittest and most consistent performer through the entirety of this season. If Barcelona do not field Frenkie-Rakitić with the intent of curbing the influence of the opposing duo, Madrid will dominate the game with ease. Only Frenkie and Raki are able to physically sustain 90 minutes against Fede should he perform as well as he has against any team he has played this season. And even if they're out there, one of them is likely to be delegated to cover the Messi hole on the right so will be busy helping out with the winger leaving Fede 1 on 1 in most situations. If he's able to win these 1 on 1s and impose himself on the game, we will be dangerous from start to finish, much like we were when Vinicius took your defense by surprise last year. This midfield battle will be key. And it is very different to last year and the year before.
2) We really cannot consider starting Isco, we need to go with wings, because your FBs and Pique tend to not give them enough respect in individual battles. If we go with the diamond 4-4-2, either our side offense will suffer horribly with Mendy and Carvajal not being polished enough pairing forced to create moves, or our defensive balance will suffer with Marcelo simply being too old to do all the jobs he needs to in that position against a top team.
We should start Bale on RW and either Vinicius/Rodrygo at LW. Either of the Brazillians is able to impose themselves in a different way. Rodrygo has a tremendous calmness and quality about him, but he tends to lack bravery, risk and unpredictability. Vinicius has that over him despite often having absolutely shocking touch and finish - he's quite simply a rather unskilled Dembele. Can be very useful on pace and chaotic style alone which breaks conventional defenses down. But yes, either is good.
Bale is a pain in the ass and not sure if lack of starts recently is a combination of mistrust/dislike from Zidane, if he's trying to piss him off to get his best version against Barcelona, or if it's just rotation to protect his calves from cracking again. But he's essential to balancing the threat, even if he walks/is ineffective 33% of the match. One cross of Bale's can result in a Benzema goal.
3) We cannot have these stroke-like defensive blunders in the first 20 minutes of either half - it's literally the worst thing that kills the momentum of this team. RM is a momentum team, a streak team, a colossus with legs of clay much like Barcelona. You can topple it, but it also has enough quality to get you if you're not careful. If we avoid conceding an early goal and prevent our offense losing confidence in creative stage due to having to chase the score from teammate's idiotic mistakes (often against the run of play...), there is a good chance this match will stay even for a long time. In Dec 2017 and the Copa Clasico it was already on course to being a draw if it wasn't for defensive brain farts.
4) Messi, Suarez and Rakitić do like playing vs Madrid, but they also have not been put against a challenge of digging Barca out of the hole very often. Copa Clasico 1st leg last year for example. If Madrid score and keep the initiative up, they can all find it quite hard to do the work if they have to work up from 0:1 or 0:2 down. Madrid are better this year. If they play like they have and do not fuck up early, this will be a different kind of Clasico for all those who saved your ass for the past few years.
Courtois; Carvajal, Varane, Ramos, Mendy; Casemiro, Valverde, Kroos; Bale, Benzema, Vinicius/Rodrygo
I still feel it'll be 2:1 or 3:1 Barca, because I don't trust our defense bar Mendy as far as I can throw them. But again, this team has a different dynamic to previous years. And a draw is also very welcome, because Barca are losing points left and right on away grounds while we have regained a bit of our depth that helped us to the 2017 title.