Sasuke would probably be initially skeptical. He'd think Naruto was up to some strange new form of training or just being his usual odd self. But if he saw that Naruto's singing of Breaking Benjamin songs actually had an effect, like inspiring the other villagers or giving them courage in a difficult situation, he might start to respect it a little more.
The common people of Camelot would be delighted. They are used to seeing Merlin as the friendly servant, and now he is singing such a beautiful song. They would gather around him, their faces lit up with joy. For the magical creatures in the surrounding forests, they would be drawn to the magic in his voice. Some might come closer to Camelot than they usually do. Some might even try to communicate with Merlin, believing that he is a kindred spirit. This could cause some chaos as the people of Camelot might be afraid of these strange creatures approaching, but also lead to a new understanding between the two worlds.
On the other hand, there would be fans who would welcome it. Maybe they never liked the pairing in the first place or they think it adds an interesting twist to the story. They could start exploring new relationships for both Sasuke and Sakura in their own fan - created stories.
Chiron would be concerned. He knows how important their relationship has been for the stability of the camp and for their own growth as demigods. He would likely try to mediate and help them work through their issues, if possible. If not, he would worry about how it will impact their future quests and the overall well - being of the camp and its inhabitants.
If Naruto was an Ajin in a fanfiction, the characters in Naruto would have a mix of reactions. Hinata would be concerned for Naruto's well - being. She might be a bit scared at first because of his new and strange abilities, but still stand by him. The villagers, who have had a rocky relationship with Naruto in the past, might be afraid of him all over again. They could see his Ajin powers as a threat. On the other hand, Naruto's close friends like Shikamaru would try to understand what's going on and help Naruto control his powers if they can.
Some characters might be shocked. For example, Naruto himself would probably be really excited to see how his world is imagined in the future. He'd be happy to see that his story has inspired so many different ideas. Sasuke, on the other hand, might be a bit more skeptical. He'd analyze the fanfiction carefully, looking for any inaccuracies or misinterpretations of his character or the ninja world's values.
Some might be amused. Naruto characters have their own set of stories and adventures, and seeing the different and often wild ideas in RWBY fanfiction could be a source of entertainment for them.
They could have a range of reactions. Naruto would likely be excited and eager to see new stories about his world. He loves adventures and would be interested in seeing different takes on his life.
Well, it could be a really unique story. Maybe Naruto, with his energetic and determined personality, would bring a new twist to Breaking Benjamin's powerful music. He might be in a world where he has to sing their songs to gain some sort of power or to defeat an enemy. For example, he could be in a battle against a powerful ninja who is immune to normal jutsu, but is affected by the emotions and energy in Breaking Benjamin's music when Naruto sings it.
They would probably be intrigued. It's a different take on their own story.
Sakura might be a bit surprised at first, but then she would be happy for Naruto. She has always been his friend and would see that he is happy with Shikamaru. Kakashi would probably be very supportive. He has seen both Naruto and Shikamaru grow up and would understand their connection. Sasuke, on the other hand, might not show much emotion on the surface, but deep down he would be okay with it as long as Naruto is happy.