In some fanfictions, Naruto might get the scar on his eye during a particularly intense battle with a powerful enemy. Maybe it was a close encounter with a rogue ninja's weapon that managed to leave a mark on his eye area.
His scar might make some people more intimidated by him. They could see it as a sign of his dangerous experiences and be a bit afraid to get too close. For example, new acquaintances in the fanfic might be hesitant to approach him at first.
Zoro often puts himself between Luffy and danger. For example, when facing strong opponents, he'll be the first to step forward and engage, making sure Luffy is safe behind him.
Hermione might be the one. With her extensive research skills and vast knowledge of magic, she could have stumbled upon a way to remove the scar. She was always determined to help Harry, and this would be just another of her great contributions.
Luffy shows his love for Nami in a very unique way. He gives her a great deal of freedom on the ship. He allows her to manage the money, which is a big sign of trust. Also, his constant protection of her is another way. For instance, during battles, he makes sure Nami is safe. He doesn't question her navigation skills and always follows her lead in that regard. His simple gestures like sharing his food with Nami also show that he cares about her in his own Luffy - like way.
It can introduce a darker and more mysterious element to the plot. For example, if Harry discovers that he has death in his scar, he might become more withdrawn as he tries to understand what it means. This could lead to him isolating himself from his friends, which in turn affects the group dynamics.