In the fanfic, Naruto could potentially have some hidden power or connection that he uses to his advantage. For example, he might discover a long - lost prophecy that says the Uchiha are crucial for the future of the village. He then uses this knowledge to persuade everyone that the Uchiha should be spared. He might also enlist the help of his friends and allies, like Sasuke, to show a united front against the idea of the massacre.
The Otsutsuki OC might be an unexpected bystander during the Uchiha Massacre. For example, they could have been on a different mission in Konoha when the massacre started. They might then decide to get involved, either to save some Uchihas out of a sense of justice or because they see potential in the Uchiha bloodline for their own purposes. So, they end up interfering with Itachi's plan and creating a whole new set of consequences for the story.
Well, the OC might try to prevent the Uchiha Massacre. Maybe they have a different view of the Uchiha compared to the Konoha leadership. They could try to convince the Uchiha to flee or negotiate with Konoha. If they have unique powers or knowledge, they could use it to expose the real motives behind the planned massacre and create a situation where the Uchiha are spared. This would completely rewrite the story of the Uchiha's downfall.
She could survive by being away from the village during the massacre. When she returns, she is shocked. She decides to investigate the real cause of the massacre. Along the way, she discovers some dark secrets about her clan and the village. This knowledge gives her a new purpose, and she uses her Uchiha powers to bring the truth to light.
One way is to make the Otsutsuki OC a hidden observer during the Uchiha massacre. They could be there to study the Sharingan or the power dynamics within the Uchiha clan. For example, they might be interested in how the Uchiha's emotions, like hatred, are related to the power of their eyes. Another way is to have the OC be a factor that somehow influences Itachi's decision. Maybe they offer him a different option or some knowledge that makes him rethink his actions, but in the end, the massacre still occurs due to the complex web of political and clan - related reasons in the Naruto universe.
The Otsutsuki OC could be a catalyst. They might have some knowledge about the future or some power that the Uchiha want. This could lead to internal strife within the Uchiha clan as some members might be tempted to side with the OC for power, while others oppose it, which could be a contributing factor to the massacre.
One interesting idea could be how the themes of sacrifice from Itachi's story in Naruto might be translated into the RWBY universe. For example, Itachi sacrificed his reputation and relationships for the greater good of his village. In RWBY, a character might have to make a similar choice between personal relationships and the safety of the kingdoms. Another aspect could be the combat styles. If Itachi were to exist in the RWBY world, how would his Sharingan abilities blend with the Dust - based combat? It could lead to some really creative fight scenes. Also, the concept of family loyalty, which is strong in Itachi's story, could be explored in relation to the families in RWBY like the Rose or the Schnee families.
It could be Sasuke's sister, if there was one in the fanfiction world. She might share some of Sasuke's traits but also have her own unique personality that attracts Naruto.
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.
She would change the story because she would be a target. Those who caused the massacre would want to finish the job. This would lead to many chase and escape scenes. Also, other characters would be divided in their opinions about her. Some would see her as a threat, while others would see her as a symbol of resistance.