I think young Sasuke would have a very intense reaction. He was so consumed by his hatred for Itachi at that young age. A Sasuke vs Itachi fanfiction would be like a trigger for him. He would be eager to see how the fictional fight is portrayed. If Sasuke in the fanfiction is shown as strong, it might fuel his own ego and make him strive even harder to become powerful. However, if the fanfiction shows Itachi as superior, it could enrage him. He might feel that it undermines his own goal of defeating Itachi and he might even want to find the writer to correct their 'wrong' view.
Naruto would probably be angry at first. He's always been a free - spirited person, and being kidnapped by Sasuke, his friend - turned - rival, would surely make him mad. He'd likely try to fight back immediately, using his ninja skills and his never - give - up attitude.
Sasuke could be intrigued. If he comes across a well - written fanfic that really captures the essence of his relationship with Naruto or offers a new perspective on his goals and journey, he might get drawn into it. He might start to think about how these different ideas could fit into his own understanding of his life and the world around him. For instance, if a fanfic shows him a different path he could have taken, it might make him reflect on his choices.
Sasuke might be in shock at first. He can't believe that Naruto is dying in his arms. He may feel a great sense of guilt, thinking that he could have done more to prevent it.
Naruto would be devastated. Sasuke is his closest friend and rival. He would likely go into a state of deep mourning at first. He might isolate himself for a while, just trying to come to terms with the loss.
He may also choose to defend himself without really fighting back. Naruto values Sakura as a friend and teammate, so he would be reluctant to harm her. He might just try to dodge her attacks and keep talking to her, hoping to snap her out of it.
Naruto's reaction would be complex. He would be devastated because his bond with Sasuke is so deep. However, he also trusts Sasuke to an extent. So, he would be torn between his immediate emotional response of anger and his long - standing faith in Sasuke. He might start to look for clues and evidence, and in the process, he could end up uncovering Kabuto's devious plan while also reaffirming his friendship with Sasuke. He would probably be more determined than ever to set things right and prove Sasuke's innocence if he is indeed innocent.
The Uzumaki family might be a bit shocked at first. But Naruto's kind - hearted nature could make them eventually come around. His mother Kushina might be more accepting as she always wanted Naruto to be happy. As for Sasuke's family, well, it's a bit more complicated. Since Sasuke is the last of the Uchiha, his family's traditional values might be a hurdle. However, if Sasuke is determined, he might be able to convince any remaining family members or those who care about the Uchiha legacy that his relationship with Naruto is a positive thing.
Another scenario could be that Naruto and Sasuke discover a new power within themselves during the fight against Kaguya. This new power could be related to their friendship or their connection to the Sage of Six Paths. They then use this power to finally defeat Kaguya.
Most likely Naruto and Sasuke. In the original series, they were already very powerful when they faced Kaguya, and in fanfiction, they could be even more so. Their combined strength and determination usually make them the favorites to win.