A challenge and a choice
In One Piece, its impossible to imagine a strawhat without a character arc. Simply because without it, the character would feel bland. In simple terms, a character arc can be reduced to a challenge that will put a protagonist into a difficult position and a series of small challenges to finally put them in front of a choice. After a specific choice is made, the character arc finishes and another might start again. A character arc means development. In means that the character will have less information about the nature of his life in the start and more information at the end. It doesn't mean necessarily that a character must change. Sometimes a character will just reaffirm in own position.
All the strawhats need a relatively strong character arc. This is a necessity to ensure the impact of the protagonists. In One piece, a strong character arc will put the Straw-hat on their final rails leading toward adventure and the integration in the crew as a real Nakama.
Usually, the character arc are concluded by the integration of the character in the crew like Sanji or Franky, but sometimes the arc comes after and the character is reaffirmed into their position like Robin, Usopp and Nami. Sometimes a character can even benefit multiple strong character arc (Sanji and Usopp for example)
We must be careful tho, because a lot of characters in One Piece have strong character arcs.
Here are some example of strong character arc by strawhats:
Zoro:
After clashing with Mihawk, Zoro had to accept the gap between his strenght, his will and his dream to go on. You could also say that Zoro had to face the responsibilities of the suffering of Luffy for his crew on thriller Bark.
Nami:
Nami had to accept to let someone enter her life to finally ask for help.
Usopp:
First Usopp had to face his fear and confront his nightmare (Kuro) at the risk of his life. Or on Water Seven, Usopp had to accept to swallow his pride in order to say "sorry" to his friend.
Sanji:
First, Sanji needed to accept leaving the baratie and leaving the old man behind. Then, on whole cake, Sanji had to accept to rely on his friend (Luffy) to save his family while risking the life of his father.
Chopper
Chopper needed to accept himself as a monster and someone worthy of having friends in order to join Luffy.
Robin
Robin had to accept to live and rely on her friend in order to survive
Franky
Franky needed to let his guilt go away and think about himself in order to join Luffy
Brook
Brook needed to accept to live another life by acepting the proposition of Luffy. His arc ended the moment he asked Luffy to join the crew.
Jinbe
Jinbe had to leave his responsibilities behind him in order to join Luffy