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.
I really like the fanfiction where the Straw Hats time travel to the future. In this story, they see how their actions have affected the world in the long run. It shows a post - apocalyptic world that was caused by a series of events that they were unknowingly a part of. It makes them reflect on their current actions and how they can change things for the better. Another good one is the one where they go back to the time of Gol D. Roger. They get to meet the legendary pirate and learn some of his secrets, which is really exciting for fans of the series.
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.
Kizaru could also be interesting. His light - based powers are extremely powerful. If he joined the Straw Hats, he could use his speed and laser - like attacks to protect the crew. However, he would need to change his loyalty from the World Government, which could create a complex and engaging storyline.
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.
The relationships will likely become more profound. For instance, Zoro might see Luffy's memories of believing in his strength even when others doubted, which could make Zoro more determined to protect Luffy at all costs.