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.
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 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.
His core personality might not change much. Luffy is still likely to be as carefree and adventurous as ever. However, he might become a bit more sensitive to certain things, like how people treat girls. He'd still be fearless in the face of danger, but could use his new form to his advantage in different ways, like when dealing with female characters who might be more likely to trust a girl.
If Luffy had a Devil Fruit that made him invisible, it could make him a bit more mischievous. He might start playing pranks on his crewmates more often. But his core values of loyalty and determination would remain. He would still use this power for good in battles, sneaking up on enemies and catching them off - guard. The new power would just add a different flavor to his already vibrant personality.
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.
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.
If Harry Potter was raised by Luffy, his personality would change significantly. Luffy is always cheerful and optimistic, and this would seep into Harry. Harry would be less haunted by his past and more focused on the present and future. His sense of humor would also be different. Luffy has a very simple and direct sense of humor, and Harry would probably adopt some of that. He would be more likely to laugh at the simple things in life. In addition, Harry would be more impulsive. Luffy often acts on impulse, and Harry would be more likely to do the same, which could lead to both great adventures and some trouble. His loyalty would also be more like Luffy's, fierce and unwavering towards his friends.
If Akainu crossed over to a more peaceful part of the One Piece world, like the Fish - Man Island, his aggressive personality might be toned down a bit. He would be surrounded by a more harmonious society, and he might start to question his extreme views. However, his sense of justice (as he sees it) would still be there, but he could become more diplomatic in his approach.
His personality might become more carefree like Luffy's. He would probably be more laid - back and not worry as much about formalities.
If Luffy was raised by Whitebeard in fanfiction, his personality would undergo several changes. He would still be the kind - hearted and adventurous Luffy we know, but with added layers. He would probably be more protective of his crewmates, not just in a fight - or - flight way but in a more paternal way like Whitebeard. His sense of justice might also be influenced by Whitebeard's own moral code. He might be less impulsive when it comes to making decisions that affect the whole crew. Instead of just following his gut instincts all the time, he would consider the well - being and opinions of his crewmates more. Also, his leadership style could be a blend of his natural charm and Whitebeard's strong leadership, making him an even more formidable captain.