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 storyline could be Naruto discovers he has Conqueror's Haki during a battle with a powerful enemy. This new power gives him an edge and he uses it to inspire his allies and intimidate his foes. For example, when facing the Akatsuki, his Haki could make some of the weaker members hesitate in their attacks.
In this kind of fanfiction, usually the plot might involve Luffy starting to discover his latent conqueror's haki power. Maybe he first experiences some intense battles where he unknowingly shows signs of it. Then, he might meet an old and wise character who recognizes this power in him and starts to guide him on how to control and use it effectively.
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.
Naruto would become more confident. With Conqueror's Haki, he would know he has an extremely powerful ability. This would boost his self - assurance in battles and in his leadership role. For instance, when leading his team, he would be more assertive.
Aside from Luffy, characters like Ace might show up in this type of fanfiction. Ace, being Luffy's brother, could have some connection to Luffy's discovery of conquerer's haki, perhaps through memories or some form of spiritual guidance. Additionally, some powerful enemies might be present. These enemies would serve as the motivation for Luffy to quickly master the conqueror's haki in order to defeat them and protect his crew. There may also be some minor characters from the One Piece world who assist Luffy in his learning process, like the inhabitants of a special village that has knowledge about haki.
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.
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, there are 'luffy haki fanfic' stories that focus on the development of his Observation Haki. In some of them, Luffy is able to predict the moves of his opponents far in advance, like being able to dodge attacks that are seemingly impossible to avoid. These fanfics often add new elements to the use of his Haki, like being able to sense the emotions behind an opponent's attack as well.