The Uchiha might initially be filled with anger. Their trust in Naruto, if it was there before, would be completely shattered. They could start to plot revenge against Naruto, believing that he has wronged them. This could lead to a lot of tense moments in the fanfiction as the Uchiha gather their strength and resources.
One great Naruto and Itachi fanfiction is 'The Bond Beyond Clans'. It delves into a scenario where Naruto discovers a hidden side of Itachi's past that was not known before. It shows how their relationship could have developed in a different way, with Naruto having a deeper understanding of Itachi's sacrifices.
In many fanfictions, the scenario of Naruto and Itachi killing the Uchiha clan is a very interesting but also controversial topic. One aspect could be the change in Naruto's character. Usually, Naruto stands for justice and protecting others. If he were involved in such a massacre, it would be a huge deviation from his original character. Maybe in the fanfiction, there could be some extreme situations that force him to do so, like being mind - controlled or having some false information.
Naruto might initially be shocked and angry. He has a strong sense of justice and being kidnapped goes against everything he stands for. He would likely try to fight back immediately, using his basic ninja skills and his determination. For example, he could try to use his Shadow Clone Jutsu to create distractions and escape.
Itachi would probably be very calm and composed. He might see Sarada as a symbol of the new generation of ninjas in the village. He could be gentle with her, perhaps sensing her potential and her desire to grow as a ninja.
Maybe the fanfic could start with the OC being experimented on by Orochimaru. Naruto and Itachi somehow cross paths with the OC. Itachi, seeing the OC's situation, decides to help, which leads to a complex relationship between them all as Naruto tries to understand Itachi's motives and the OC tries to break free from Orochimaru's control.
One common theme is redemption. Itachi, with his past actions, sees protecting Naruto as a way to redeem himself. For example, in many fics, he tries to make up for the wrongs he did to his clan by ensuring Naruto's safety, as Naruto represents hope for the village.
Another theme is redemption through love. Itachi has done some morally ambiguous things in the Naruto canon. In fanfics, his love can be a path to redemption. His love for someone might make him question his past actions and strive to make up for them. This can create a very interesting character arc within the fanfic.
First, you need to develop your OC well. Give them a unique backstory, personality, and goals. For example, if your OC is a ninja, what are their special abilities? Then, think about how they interact with the existing characters. With Orochimaru, it could be a relationship of fear or mentorship. With Itachi, perhaps respect or a family - like bond. For Naruto, it could be friendship. Also, have a clear plot. Maybe the OC is caught in the middle of a power struggle between Orochimaru and those who oppose him, and Itachi and Naruto are on different sides of this struggle.
In some fanfictions, a possible motivation for Naruto could be to prevent a greater evil. Maybe he was tricked into believing that the Uchiha clan was about to start a war that would destroy the entire ninja world. So, he thought that by taking part in killing the clan, he could save more lives in the long run.