Caesar Clown is a former colleague of Vegapunk and the leading expert on chemical weapons of mass destruction.
Caesar’s dream is to surpass Vegapunk and become the greatest scientist in the world. Luffy’s crew at the moment doesn’t have a scientist it would make sense if Caesar became the Straw Hat scientist.
Caesar at the moment is hated by most of the Straw Hats but this could all change if Caesar decides to chose a new path and realise that what he did in the past is wrong and help out Luffy and the crew in some way.
Nefertari Vivi is the princess of Alabasta.
Vivi traveled with and assisted the Straw Hhat Pirates for almost the entire Alabasta Saga and strongly considered joining their crew after Crocodile’s defeat. Though she ultimately declined their offer in favor of working to restore her country, she remains closer to the Straw Hats than anyone else outside of their crew. She and Carue are now regarded as honorary members of the Straw Hat Pirates, at least by Luffy and the others. In the future we might see her joining up with the Strawhats.
#Marco The Phoenix
Marco the Phoenix is the former 1st division commander of the Whitebeard Pirates. He was Whitebeard’s closest confidante and his right-hand man. After the Payback War, Marco works as a doctor at Whitebeard’s home village.
The last order that Whitebeard gave him was to protect Luffy while in Marineford war so the best way to ensure that Luffy lives is by joining the crew.
Marco would boost/leverage Luffy crew’s fighting power since they will need to face Yonkos.
Bartolomeo is the captain of the Barto Club and the commander of the 2nd division of the Straw Hat Grand Fleet.
Bartolomeo, an original admirer of Luffy from the time of Loguetown and onward, admires the crew in its entirety, saying he followed all of their actions.
What job the Straw Hat Crew actually requires is a Guard who basically protects the ship during their adventure in various islands in the New World. This is emphasised by the fact that in almost every arc, the crew splits up and somebody is left behind to protect the Sunny. With Bartolomeo’s Barrier Fruit this problem can be solves if he creates a barrier around Sunny and the crew can go adventuring into the islands.
O-Tama is a girl from the region of Kuri in Wano Country. She is a kasa weaver and a kunoichi in training who follows the Kozuki Family.
When Ace visited Wano, Tama formed a bond with and had faith in him. Tama wanted to go out to sea with Ace, but he told her that she was too young at the time. He promised that he would take her with him if she became a kunoichi by the time he returned and Tama waited for him since.
What if Luffy keeps Ace’s promise and takes O-Tama with him?
Carrot is a rabbit mink and a member of the Inuarashi Musketeer Squad. She joined the Sanji Retrieval Team to rescue Sanji on Whole Cake Island.
Carrot is loved by everyone and has the potential of becoming a Strawhat crew member. She’s very strong and has developed a great relationship with the Strawhat Pirates.
She could be the watch on top of the boat (we’ve seen her at this position multiple times already). Like the other Straw Hats members she has a dream, and it is to see the world. She’s a female character and it’s been a long time since the crew got a female character in the mix (I really believe there will be a third woman in the crew before the end of the journey).
Mr. 2 Bon Kurei, is a former officer agent of Baroque Works and an okama. While he was once an enemy of Luffy’s as a member of Baroque Works, the two became quick friends, leading Bon Kurei to sacrifice himself for Luffy both at Alabasta and later at Impel Down.
He is currently in Impel Down not as a prisoner but as Okama Queen in level 5.5 . There is a high possibility of him joining Luffy in future.