In Wano, we had Zoro and Luffy team up together to face down Kaido: during this fight, in order to protect Luffy, Zoro unlocked CoC officially, and used it to scar Kaido.
Oda is setting up Sanji to do the same thing with Big Mom: this was why Black Maria's section was so important, it's showing Sanji's growth in terms of realizing women (like Robin) can't be treated as weak and helpless. Real men, like Zoro and Sanji, trust strong women like Robin and Vivi to handle themselves.
Sanji's Will has always been held back because he can't attack women. Let's look how this has been handled:
-Enies Lobby: Sanji is getting beaten up by a woman, and has to be rescued by Nami, another woman. Sanji is forced to allow Nami to give him aid.
-Wano: Sanji is getting beaten up by a woman, and has to choose to rely on a woman, Robin, for help, by putting her in danger.
In Wholecake, Sanji was willing to beat up Luffy in order to protect him from Big Mom. However, Sanji has never been forced to choose between protecting a crewmate and attacking a woman. Sanji will go from rejecting help, to asking for help, to overcoming his weakness.
With all the connections between Big Mom and Sanji, and how much pain she caused him, it makes sense for Big Mom to be the first woman Sanji fights. If Sanji has to choose between watching Luffy die, or kicking Big Mom... he's going to unlock CoC, and overcome his weakness. I could also see Sanji unlocking CoC by protecting Pudding from Big Mom.
(This theory obviously assumes Big Mom is still alive. Also, Vegapunk will play a big role in making Sanji realize who he is, too, I think. Vegapunk explaining how Judge modified his kids is going to be interesting).