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.
There could be a 'naruto akainu fanfiction' that focuses on a team - up between Naruto and Akainu. Maybe Naruto, with his belief in redemption, tries to change Akainu's extreme views while Akainu is initially resistant but gradually starts to see things from Naruto's perspective. This would create a lot of interesting character development and conflict.
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.
One idea could be a crossover with the 'One Piece' world where Akainu has to team up with pirates to fight a common, greater evil like an ancient sea monster. This would create a lot of tension as Akainu is known for his hatred towards pirates.
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.
Superman from the DC universe. Superman stands for truth, justice, and the American way, but his methods are often more lenient compared to Akainu. The two could have a philosophical debate about justice while also having to fight a common enemy.
If Akainu crossed over to a more peaceful part of the One Piece world, like the Fish - Man Island, his aggressive personality might be toned down a bit. He would be surrounded by a more harmonious society, and he might start to question his extreme views. However, his sense of justice (as he sees it) would still be there, but he could become more diplomatic in his approach.
Well, Akainu could cross over into a fan - made version of the One Piece world where the Yonko are all united against the marines. He finds himself in the middle of this conflict and has to decide whether to side with the marines who he used to be a part of or join the Yonko for some self - interested reasons. He could also have a change of heart during this crossover and start to see the pirates' side of things, which would be a huge character development for him considering his strong stance as a marine in the original story.
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.