STEEV

Vivi can't be the final member of the straw hats, because she's the 7th member. She didn't leave the crew; she just had to stay behind to help her country recover. That was the whole dang point of the armband farewell scene - reaffirming that she was nakama and they would reunite in the future. Contrast with Yamato, who also stayed behind to help rebuild her country, but explicitly said she would not join until she was ready.