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.
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 the fanfiction scenario where Dragon saves Luffy, it could start a whole new arc. Luffy might be injured badly when Dragon arrives. Dragon could then take Luffy to a secret revolutionary army base to heal. During this time, Luffy could meet other revolutionary army members who could teach him new skills or give him knowledge about the secrets of the Grand Line or the Void Century. This encounter could also strengthen Luffy's resolve to become the Pirate King and at the same time, make him more aware of the larger political struggle going on in the world of One Piece.
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.
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 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.
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.
Well, he could change a lot of things. Maybe he'd prevent some bad events from happening in the past. For example, he could stop Frieza from destroying Planet Vegeta earlier. It would be really cool to see how his super saiyan power affects the past timeline.
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.
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.
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.