Well, as of the latest manga chapters, Yamato has indeed become a part of the Straw Hats. This decision brings new dynamics and adventures to the crew.
At first, not very well. The Straw Hats have had bad experiences with Marines. They might see the Marine OC as a potential threat. However, if the OC shows that they are different from the typical corrupt Marines, like by helping the crew out of a tough situation right away, they might start to be more accepting.
In a fanfiction where Smoker joins the Straw Hats, it could be really interesting. Smoker is a marine, and his sense of justice is quite different from the Straw Hats' view of freedom. At first, there would likely be a lot of tension. But as time goes on, he might start to understand their way of life. For example, he could see how they help the weak and oppressed in their own unorthodox way. He might also learn to be more flexible in his views of right and wrong, as the Straw Hats often operate outside the normal rules. And his combat skills would be a great addition to the crew, making them even more formidable in battles against stronger enemies.
In a fanfiction where Naruto joins the Straw Hats, the dynamic of the crew would change significantly. Naruto is used to working in teams in his own world, and his experience in forming strong bonds with his comrades would blend well with the Straw Hats' own sense of camaraderie. His ability to make friends easily would also help him fit right in. Plus, his shadow clone jutsu could be used for various tasks on the ship, like helping with repairs or keeping watch.
Well, it could be really interesting. There would probably be a lot of initial distrust from the Straw Hats, since they are usually against the Marines. But if the OC has a good heart and a strong reason for leaving the Marines, like disagreeing with their corrupt ways, the Straw Hats might gradually accept them. The OC could bring in some unique skills and knowledge about the Marines' operations, which could be useful for the crew in various situations, like getting out of tight spots when facing other Marine forces.
Popular fanfiction often has strong character development. For example, it might explore Luffy's leadership in more depth or show how Zoro overcomes a new kind of sword - related challenge. Another characteristic is unique plot twists. Maybe the Straw Hats find a way to reverse a Devil Fruit power in a fan - made scenario. Also, good fanfiction keeps the essence of the original One Piece world, like the importance of friendship and the pursuit of dreams within the crew.
If Smoker joins the Straw Hats in fanfiction, the development might be slow. At first, he would struggle to fit in with their laid - back and often rule - breaking ways. But his sense of justice could lead him to find common ground. For example, he might see that the Straw Hats are fighting against corrupt forces, just in a different way than the Marines. As the story progresses, his character could evolve. He might start to see the world from the Straw Hats' perspective and become more accepting of their methods. This would not only change him but also affect how the Straw Hats view the Marines in general.
One possibility is that they gain a deeper understanding of Luffy's motives. For example, they might see his memories of making the promise to Shanks and realize how important it is for him to become the Pirate King. This could strengthen their resolve to support him even more.