Luffy could change the past in several ways. Firstly, he could use his devil fruit powers in a more strategic way. Since he has the experience of future battles, he knows his opponents' weaknesses better. So, when facing the enemies who were involved in Ace's capture, he can defeat them more easily. Secondly, he could try to influence Ace's relationships. Maybe he can make Ace form stronger alliances with other pirate crews, which would make it more difficult for the Navy to target him. Also, Luffy could try to expose the Navy's secret plans earlier, which would change the power dynamics in the One Piece world and ultimately save Ace.
In this fanfiction, Luffy would change the past by being more proactive. In the original timeline, things happened that led to Ace's death. But now, Luffy has a chance to rewrite that. He could start by training harder in the past and teaching Ace some new fighting techniques he learned in the future. He could also interfere with the politics of the pirate world. For instance, he might try to unite more pirate crews against the common enemy, the Navy, earlier. By doing this, the Navy would be more cautious in their actions against Ace. And Luffy could also change the past by preventing some internal conflicts within the Whitebeard Pirates that weakened them and made Ace more vulnerable in the first place.
One possible plot twist could be that when Luffy goes back, he discovers that Ace's fate is more intertwined with the World Government's secrets than he thought. Maybe Ace was targeted not just because of his bloodline but because he knew something dangerous. Another twist could be that in the past, Luffy has to team up with unexpected allies, like some of the former enemies of the Straw Hats, to save Ace. And perhaps, his time - traveling disrupts the timeline in a way that creates new threats not only to Ace but to the entire world of One Piece.
His leadership style could change significantly. In the original, he leads with his heart and by example, but in a dark fanfiction, he could be more of a dictator - like leader within his crew, making decisions without much input from others and demanding strict obedience. His goals might also shift from just finding the One Piece and having a grand adventure to seeking power for more self - centered reasons, like revenge or dominance.
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 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.
Well, in most cases, Luffy would use his unique abilities. He might use his Gum - Gum Pistol to knock out the bad guys. His superhuman strength and speed would allow him to reach Nami quickly and stop the assailants before any harm could be done. Luffy's sense of justice and loyalty to his crewmates are so strong that he would not let anything bad happen to Nami.
Well, 'Luffy and Ace: Unbreakable Ties' is a fanfiction that I really like. It focuses on their childhood memories and how those shaped their relationship as they grew up. There's also 'Ace and Luffy's Great Escape' which is full of action. It shows their cleverness in getting out of difficult situations. 'Luffy and Ace: The True Friendship' is a must - read too. It simply and effectively shows the depth of their friendship.
The Dark Dark Fruit could enhance Luffy's physical strength in a different way. Instead of just relying on his rubber body, the fruit's power could give him a more menacing presence in battle. He could use the darkness to absorb attacks and then redirect that energy, making his counter - attacks even more powerful. For instance, if an enemy throws a punch, Luffy could use the dark power to suck in the force of the punch and then send it back double - strength.
Ace would probably be angry at first. He doesn't want Luffy getting involved with such a dangerous person like Blackbeard.
In a fanfiction, Ace's own experiences and battles could be related to Luffy's metabolism. Maybe Ace has some knowledge about a special fruit or power that could enhance Luffy's metabolism. He could share this with Luffy, leading to a new adventure where Luffy tries to obtain this thing to boost his metabolism. Also, Ace's death could have an impact on Luffy's metabolism. Luffy could go into a state of shock or extreme grief that either slows down or speeds up his metabolism in an unexpected way.
Luffy being raised by Doflamingo would be a radical departure from his normal upbringing. Doflamingo's way of life, which involves a lot of underhanded tactics and a disregard for the well - being of others, would shape Luffy in a very different way. He may not have the same innocent and carefree attitude. His goals could be more self - centered, like taking over territories or amassing power for the sake of it. His relationships would also be different. Instead of forming deep bonds of friendship, he might form alliances based on power and convenience. And his approach to challenges would be more in line with Doflamingo's cut - throat style, rather than his own original tenacious but more morally - guided approach.