Luffy and Ace's interaction is always key. In fanfiction, it could be that Ace reveals a secret to Luffy right before the final battle that gives Luffy newfound strength. Another great interaction could be between Luffy and Garp. Garp might be conflicted between his loyalty to the Marines and his love for his grandsons, and this inner turmoil could lead to some really emotional and tense moments when he interacts with Luffy at Marineford.
One possible plot could be that Gear 5 Luffy arrives at Marineford during the big battle. He might use his newfound powers to turn the tides of the fight. For example, he could use his rubbery abilities in even more extreme ways, like stretching to deflect cannonballs aimed at his allies and then bouncing them back at the enemy ships. His presence would shock both the Marines and the pirates. He could also engage in a one - on - one fight with some of the top Marine officers in a more comical yet powerful way, as Gear 5 gives him a more carefree and powerful fighting style.
One possible plot development could be that his death shatters the Straw Hat crew. They might disband in grief, each going their own way. Zoro could become consumed with revenge, vowing to take on the Marines who were responsible.
Well, in this fanfiction, Luffy could interact with the Marines in a more diplomatic way. Since he has the advantage of time travel, he might understand the reasons behind the Marines' actions better. So, he could try to negotiate or make deals with some of the more reasonable Marines. He could offer to help them with other threats in the One Piece world in exchange for them leaving his friends alone at Marineford.
A possible plot is that Luffy discovers a time - traveling device at Marineford. He then uses it to go back in time briefly to prevent some of the worst events from happening, like Ace's capture. Or perhaps Luffy has a dream where he is given advice by a long - dead pirate on how to win the battle at Marineford. This advice then helps him make better strategic decisions during the real battle. He could also meet a mysterious character at Marineford who has the power to heal all the wounded, which changes the whole course of the battle.
Well, in the fanfiction, Gear 4 Luffy might have had some new and unexpected interactions. Maybe he could have teamed up with some other pirates in a different way. Since he was in such a powerful form, he could have inspired more of the pirates to fight back more bravely against the Marines. And his presence in Gear 4 could have also affected the admirals' strategies, making them more cautious or forcing them to use their more powerful techniques earlier.
Perhaps in this fanfiction, Ace, Luffy and Sabo will make Marineford a place where pirates and marines can live in harmony. They could set up some rules that both sides can abide by.
One possible plot twist could be that Luffy's time travel alters the fates of some of the characters who died at Marineford. For example, he might be able to save Ace. Another could be that his presence in the past changes the future relationships between the pirates and the Marines in unexpected ways. Maybe some pirates who were once enemies now become allies because of Luffy's interference.
In this fanfiction, Ace, Luffy and Sabo could have a future in Marineford that is full of hope. Maybe they work together to change the perception of pirates in that place. Ace could use his fire power to protect the weak, Luffy with his unwavering will continues to inspire others, and Sabo with his intelligence devises plans to make Marineford a more inclusive place for both pirates and marines.
Nami would probably be heartbroken. She has been with Luffy through so much and his death would leave her feeling lost and alone. She might even lose her will to continue sailing at first.