One possible plot could be that Akainu attacks the Straw Hat's new island, endangering Luffy's friends. Luffy, filled with rage, challenges Akainu. After an intense battle, Luffy discovers a new form of his Gear ability that allows him to withstand Akainu's magma and finally defeat him.
Considering Luffy's character development, he has always been against those who harm his friends and the innocent. Akainu has caused great harm, especially during the Marineford War. If Luffy continues to grow in strength, and if the story progresses in a way that pits them against each other again, there's a chance. But the Marines also have their own ways of protecting their top - ranked officers, so it's a complex situation.
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.
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.
Luffy will most likely use his Gum - Gum abilities. He could stretch his arms to grab Nami and pull her away from the abuser quickly. For example, he might use Gum - Gum Pistol to knock out the bad guys who are abusing Nami.
One common theme is the clash of ideologies. Akainu has a very strict and uncompromising view of justice in his original story, while Naruto has a more forgiving and rehabilitative view. So when they interact in fanfiction, this clash often forms a central theme.
One good Admiral Akainu fanfiction could be 'Akainu's Redemption'. It might explore his character in a new light, showing his journey towards a different perspective. Another could be 'Akainu in the New World' which could detail his adventures in uncharted territories. There's also the possibility of 'Akainu's Hidden Past' that delves into his backstory and what made him the person he is in the original story.
Well, it could be about Naruto having to deal with the strict and fiery nature of Akainu as his father. Maybe Naruto has to adapt to a more military - style upbringing while still holding onto his own ninja values.
I'm not entirely sure what about this 'One Piece' fanfic has made you furious. Maybe the author took some liberties with Akainu's character that you didn't like? For example, if they made him act out of character in a way that didn't seem true to the original 'One Piece' story.
It could be that the fanfic misrepresented Akainu's power levels. In 'One Piece', Akainu is a very powerful character, and if the fanfic made him seem weak or overpowered in an unrealistic way, that would be annoying.