In a fanfiction where Naruto becomes like Itachi, it could be due to intense training. Naruto might study Itachi's fighting techniques, like his use of the Sharingan-based moves. He could also adopt Itachi's calm and collected demeanor, which is very different from his usual energetic self. For example, he may start to think more strategically in battles, just as Itachi did.
Perhaps in the fanfiction, Naruto has a life - changing encounter. He could meet a character who knew Itachi well and teaches Naruto about Itachi's values. Naruto, being the determined person he is, decides to incorporate those values into his own life. So he starts to act more like Itachi in terms of his self - sacrifice and his sense of duty towards the village. He may also develop a more refined and precise fighting style similar to Itachi's.
If Naruto becomes like Itachi in fanfiction, it would change his relationships. His friends might be a bit shocked at first. For example, Sasuke's perception of Naruto would be completely different. Naruto would also have a different role in the village. He might be more involved in the shadowy aspects of protecting the village, like Itachi was. And his enemies would have to re - evaluate their strategies against him.
One reason could be for strategic purposes. Itachi was a highly strategic ninja. So, in fanfiction, Naruto might act like him to outwit a particularly difficult enemy. For instance, when facing an enemy who is a master of deception, Naruto could adopt Itachi's calm and calculating demeanor to see through the enemy's tricks.
It could also be that Naruto has been influenced by Itachi's self - sacrifice. He saw how Itachi sacrificed everything for the greater good. So, in the fanfiction, Naruto might secretly work for Itachi as a way of carrying on his legacy. He may be helping Itachi from the shadows to achieve goals that Itachi was unable to complete during his lifetime.
One possible plot twist could be that Naruto, after learning more about Itachi's sacrifices for the village, starts to idolize him in a way that changes his own ninja path. He might adopt some of Itachi's stealthy and strategic techniques. For example, he could focus more on gathering intelligence before acting, rather than his usual head - on approach.
Another plot could be that Naruto is tricked into believing a false narrative about the ninja world. Similar to how Obito was manipulated by Madara's lies. He then starts to act in ways that are contrary to his usual self. He might target the other villages, thinking he is doing the right thing for some grand, but misconstrued reason. This would lead to him being at odds with his former friends, who would be desperate to bring the old Naruto back.
In a fanfiction context, it could be possible through some creative plot devices. Maybe a powerful jutsu was used that altered their memories or destinies. For example, a rogue ninja with a forbidden time - bending jutsu made them believe they were in love and got married. It could also be due to some sort of political alliance gone wrong in a strange way, where they were forced into a marriage - like situation and over time, developed real feelings for each other.
One possible motive could be to use Naruto as a bargaining chip. Itachi was a complex character with his own goals related to the safety of the Hidden Leaf Village in some ways. He might have thought that by kidnapping Naruto, he could influence the village's decisions. For example, he could try to stop certain actions the village was planning that he believed would lead to more harm. Another motive could be related to the Nine - Tails within Naruto. Itachi might have wanted to study or control the power of the Nine - Tails for his own purposes, perhaps thinking it could be a way to protect Sasuke in a different manner.
One possible plotline could be that Naruto is forced into a situation where he has to kill Itachi to protect the Hidden Leaf. Maybe Itachi is being controlled by some dark force and Naruto, with great reluctance, has to end his life to save the village and his friends.
In fanfiction, the idea of Naruto being Itachi's half - brother might be based on some creative reinterpretation of the clans. Suppose there was a long - lost connection between the Uzumaki and the Uchiha clans. This could potentially lead to a situation where Naruto and Itachi share a half - brother relationship. However, it's important to note that this goes against the canon story. The beauty of fanfiction is that it allows for these kinds of 'what - if' scenarios.
In fanfiction, the idea of Naruto having a son with Itachi might be part of a complex AU (alternate universe). The story could set up a world where the Uchiha and Uzumaki clans have a different relationship. Itachi, instead of following his original path, gets involved with Naruto in a way that leads to the birth of a son. This could be due to political alliances, or a new prophecy in this alternate Naruto universe that binds their fates together.