If Luffy goes back far enough to change the events at Water 7, perhaps Usopp would not leave the crew. This would prevent the internal conflict that the crew faced at that time. The crew would be more united from the start, and they might be able to face Enies Lobby even more effectively. Their bond would be stronger, and it could lead to different power - up scenarios for the crew members as they would be more focused on growth together.
Suppose Luffy goes back to the time before he even started his journey and changes some events in East Blue. He might be able to recruit stronger crew members from the start. He could also prevent some of the tragedies that his future crew members faced in their past. This would lead to a more confident and perhaps more powerful Straw Hat crew right from the beginning of their grand line adventure.
One possibility could be that Luffy goes back in time and decides to prevent the death of Ace. He might use his knowledge of future events to stop the Marines from getting to Ace in the first place. This would change the entire course of the story, as Ace's death was a major turning point for Luffy and the Straw Hat crew.
The crew would be really curious. They might see Luffy's father as a powerful ally. It could also boost their morale, knowing that Luffy has such a strong family connection.
In such a fanfiction, Luffy could potentially prevent some of the tragedies that occurred in the original story. For example, he might be able to save Ace from his untimely death. He could also use his knowledge of future events to form stronger alliances earlier and gain more powerful allies. This could lead to a much more powerful Straw Hat crew and a different power balance in the One Piece world.
If a marine joins Luffy's crew in a fanfic, it could be quite interesting. At first, there might be a lot of distrust among the crew members. After all, they've been fighting against marines for a long time. But if the marine has a strong sense of justice that aligns with Luffy's own view of right and wrong, they could gradually be accepted. For example, the marine might bring in some new combat techniques or knowledge about the navy's operations that could be useful to the Straw Hats.
His personality might be more serious in some ways. Since he has the knowledge of the future, he might be more cautious about certain actions. But at the same time, he'd still be the fun - loving Luffy. For example, he'd still love meat, but he might be more strategic when approaching battles or dealing with powerful enemies, like Blackbeard.
Naruto's outgoing personality would mean he'd quickly make friends with all the members of the One Piece crew. His ability to see the good in people, like how he saw the good in Gaara, would help him understand and get along with characters in One Piece who might seem rough on the outside, like Sanji. He would also bring his own brand of leadership. While Luffy is the captain, Naruto could inspire the crew in different ways, perhaps by sharing his own tales of growth and perseverance, which could give the crew new perspectives on their own quests.
On the other hand, it could potentially cause some friction. If their possessiveness leads to them being overprotective of each other, it might interfere with crew decisions. For example, if a mission requires one of them to be in a risky situation, the other might object due to their possessive feelings, which could create some internal conflict in the crew.
The Straw Hat crew would have a mix of reactions. Franky might start building a special nursery on the ship, using all his engineering skills to make it safe and comfortable. Robin would be a source of calm and knowledge, sharing stories about pregnancies in different cultures she's read about. Brook would be playing gentle music to soothe Nami. And Luffy, as the father, would be constantly by Nami's side, sharing his positive energy and telling the baby stories about their adventures, even though the baby can't hear them yet. Overall, the crew would come together to support Nami and the new life coming into their world.
Time travel would have a massive impact on the One Piece storyline in fanfiction. Imagine if a time - traveler went back and influenced the actions of the World Government in the past. This could lead to a completely different present. Maybe certain Devil Fruits would be in different hands, or certain islands would have different rulers. The relationships between the different pirate crews would also be affected. For example, if the Whitebeard pirates had different experiences due to time travel interference, their relationship with the Straw Hats could be altered in many ways. It would be like a domino effect, with one change in the past causing multiple changes in the present and future of the One Piece world.
Female Luffy's personality would bring a fresh energy to the Whitebeard crew. If she has a similar sense of humor to the male Luffy, she would keep the crew laughing during long voyages. Her impulsiveness could lead to some unplanned but exciting adventures for the crew. Also, her empathetic side, which might be more pronounced as a female, could help her better understand the emotional needs of the crew members. For example, she could be more in tune with a crew member who is homesick or feeling down, and be able to cheer them up in a more nurturing way than the male Luffy might.