It might have been CoC, who knows, but it's true that Luffy and Zoro weren't overly impressed. I guess that is the result after seeing Kaido, Big Mom and the Gorosei the past few weeks.
If we go by the prerequisites I'd wager Sanji has to find something he wants to conquer. Luffy wants to be PK and protect his crew. Zoro wants to be WSS and protect Luffy. What does Sanji want to conquer?
As of now he wants to find All Blue and protect the ladies. Apparently not enough. Like Zoro had to recognize that he had to become a conqueror in order to protect Luffy I expect Sanji has to be willing to give in to Science and understand he too needs to become a conqueror so that he can actually help Luffy and Zoro when the going gets tough even for them. In other words he needs to embrace the leadership so he can become a worthy left arm/wing.
The best way would be against a crew/WG where he knows that Luffy and Zoro already have their hands full with #1 and #2 and there is a #3 that is also a huge threat. If he dicks around himself and everybody else including Jinbei and downwards are doomed. Ryukogyu would be very fitting if Luffy took on Akainu and Zoro Fujitora/Kizaru.
