Zoro maybe simple but he has better natural development than Sanji without even needing an arc.
By the pre time skip, he had one of the best developments in the show. We saw Zoro going from a lone wolf to be loyal follower and simultaneously making Luffy a better leader. There is another aspect of Zoro’s character that is him developing into a proper samurai/bushido and Wano is the perfect place to represent that. Zoro represents eastern masculinity. He is honourable, a loyal follower, kind(Felt for Yasui and people of okobure town), levelheaded (In Zou he reminded Luffy of dangers of going to wci and its repercussions), Zoro is the epitome of self control and discipline, protector of weak people. And Oda highlights these traits of Zoro again and again throughout the story unlike vague mentions of Sanji being Kind etc.
