One possible thing that could happen is that Roger would tell Luffy about the real origin of the Devil Fruits. Given that Roger had explored so much of the world, he might know secrets about these strange fruits that no one else does. This knowledge could give Luffy an edge in his battles and his journey. Another aspect could be that Roger shares his experiences with the ancient weapons. Luffy might then be more aware of the power and danger they pose, which could be crucial in his future adventures.
If Luffy meets Roger, Roger could introduce Luffy to some of his old crewmates who are still alive. These crewmates could teach Luffy new skills or tell him more stories about Roger's adventures. For example, they could show Luffy how Roger used to navigate through the toughest of waters. This would not only help Luffy become a better pirate but also strengthen his resolve to follow in Roger's footsteps. And maybe, just maybe, Roger would tell Luffy where he hid the last road poneglyph.
Well, in such a fanfiction, Luffy might gain some valuable advice from Roger. Maybe Roger could share his experiences about the One Piece and the Grand Line, which would inspire Luffy further in his journey to become the Pirate King.
In a fanfiction where Luffy meets Roger, it could be an amazing encounter. Roger might see in Luffy the same spirit of adventure and freedom that he had. He could pass on some hidden knowledge about the One Piece or give Luffy a cryptic hint about the true nature of the Grand Line. This meeting could also inspire Luffy even more, fueling his determination to become the Pirate King. It would be a moment full of emotion and significance for both characters and for the entire One Piece universe.
Luffy traveling to Roger's time would be chaotic. He'd probably try to challenge Roger to a fight right away, seeing as he challenges everyone strong. But Roger would likely see something special in Luffy. Luffy might also introduce some new ideas to that era. Since he's from a more modern time in the story, he could have knowledge about things like different fighting techniques that haven't been developed yet in Roger's time. This could disrupt the balance of power in that era.
In this fanfiction, perhaps Luffy would get some valuable advice from Gol D Roger. Roger could share his experiences about the One Piece and the Grand Line, which might give Luffy new insights on his journey to become the Pirate King.
In the official setting of One Piece, Luffy was the son of the revolutionary Dragon and the grandson of Vice Admiral Karp. He had no direct father-son relationship with One Piece Roger. However, there were also some unofficial analyses that believed that Luffy might be Roger's son. Judging from the way the Roger Pirates treated Luffy and Ace, during the war, Ace was captured by the navy and was about to be executed. The crew of the Roger Pirates did not go to the battlefield to save him, but Luffy received a lot of help during his growth, such as the guidance of the red hair, the help of cats and dogs in the country of peace, the help of Baki in the underwater prison, and Riley saving Luffy and teaching him how to be domineering. Furthermore, the expensive devil fruit like the rubber fruit might have been kept by Roger and given to Luffy when the time was right. The red-haired man even gave Roger's straw hat to Luffy. At the same time, Roger entrusted the child to Karp. Karp might have swapped Ace with Luffy, and Luffy and Roger had the same dream and will, and they looked similar. However, these were only speculations and analysis by some fans, not official conclusions.
The first thing that would likely happen is that the Straw Hat Pirates would immediately start to track down Luffy. They have a strong bond, and they wouldn't just let their captain be held captive. They would use all their skills and resources to find him.
Well, they could be on a mission to find a special treasure. Luffy is all gung - ho about it, and Zoro is just following his captain's lead. Along the way, they face many obstacles, but their friendship keeps them going. For example, Zoro might get injured protecting Luffy, and Luffy, in his own goofy way, tries to cheer Zoro up and make sure he heals quickly.
In a fanfiction where Luffy turns into a girl, the story could take an interesting turn. Luffy's dreams of becoming the Pirate King wouldn't change, but the way he approaches it might. As a girl, he could use his charm to get out of some tricky situations. For example, when dealing with some of the more traditional pirates or marines who might underestimate a girl. Also, the Straw Hat crew's dynamic would shift. Zoro might be more protective, not that he isn't already, but in a different way. And Chopper would be super curious about the sudden change and would constantly be checking if there are any medical reasons for it, even though it's a magical fanfiction change.
In a fanfiction scenario where Luffy meets Whitebeard, it could be a really epic encounter. Luffy, being the carefree and determined pirate he is, might immediately show his respect for Whitebeard as a great pirate. Whitebeard, on the other hand, would probably be intrigued by Luffy's unwavering spirit. They could end up sharing stories about their adventures at sea, and perhaps Whitebeard would offer some words of wisdom to Luffy. Maybe they would even team up briefly to take on a common enemy, like some other powerful pirate crew that threatened the peace of the Grand Line.
In a fanfiction where Luffy meets Dragon, it could be an emotional moment. Luffy might be shocked to learn that Dragon is his father. Dragon could impart some deep wisdom or a special mission to Luffy. Maybe he would reveal secrets about Luffy's past or the world that Luffy didn't know before. It could also lead to Luffy having a new sense of purpose in his journey to become the Pirate King.