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.
The plot might involve Merlin singing 'Thousand Years' as a spell. He could be in a situation where Camelot is in great danger, like being attacked by a powerful dark magic. Singing this song, which is full of emotions of love and endurance, he could be converting the negative energy into positive. His voice could reach every corner of Camelot, protecting the people and the castle from the dark forces, and at the same time, it might also touch the hearts of the knights and Arthur, making them realize how much Merlin truly cares for Camelot.
It can deepen their bond. The song is full of love and longing, so when Merlin sings it to Arthur, it makes Arthur see Merlin in a new light. He may start to realize the depth of Merlin's feelings for him, whether it's platonic or something more.
In some fanfictions, the use of 'A Thousand Years' by Merlin singing to Arthur is set in a modern - day Arthurian reincarnation scenario. Merlin is a singer in a small club, and Arthur, who has no memory of his past life, is drawn to Merlin's performance of the song. As the story progresses, they slowly start to remember their past connection.
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.
If Naruto was caught singing fanfiction in the village, there would be a whole range of reactions. The merchants in the village might see it as a new opportunity to create merchandise related to this new 'Naruto - singing - fanfiction' thing. The ninja families might be more concerned about whether this is a sign of Naruto getting distracted from his duties. The children would likely be the most excited, running around and telling everyone about it. The village elders, on the other hand, would probably have a long discussion about whether this is an appropriate behavior for someone of Naruto's standing. But in the end, Naruto's charm and popularity would probably smooth over any major negative reactions.
Arthur might be initially shocked and confused. He's used to Merlin being in control. But then he might find it quite hilarious and start teasing Merlin about it.
She might be completely overjoyed. With tears of happiness in her eyes, she'd immediately say yes and throw her arms around Merlin. After all, she loves him deeply and has probably been waiting for this moment.
He might also get angry. Merlin has his own pride and after all he has done for Arthur, being insulted like this would make him furious. He could start yelling back at Arthur, listing all the times he has saved Arthur's life or the kingdom.
He might be surprised. The Harry Potter world has its own set of rules and characters that are very different from what Merlin is used to in his Arthurian world.
Merlin would probably be very conflicted. On one hand, he is loyal to Arthur and Camelot, but on the other, being forced into a marriage is a huge infringement on his freedom. He might be scared about how this will change his life and his ability to use his magic secretly. Arthur, on the other hand, could be initially resistant as well. He sees Merlin as a friend and a servant, and this sudden change to a marital relationship would be strange for him.