One possible plot could be that Naruto has finally had enough of Sakura's constant nagging or her one - sided focus on Sasuke. So he tells her off in a moment of exasperation, perhaps pointing out how she's been neglecting their friendship or the team spirit in pursuit of Sasuke.
It could be that Sakura has been treating Naruto poorly for a long time, underestimating his abilities or always putting him down in comparison to Sasuke. Naruto, after achieving some great feats, decides he won't take it anymore and tells her off. He might tell her that he has his own dreams and strengths and doesn't deserve to be treated that way.
Maybe Sakura makes a big mistake during a mission, like endangering the whole team because of her emotions towards Sasuke. Naruto then tells her off, saying that she needs to grow up and think about the greater good rather than just her personal feelings. He might remind her of their ninja code and how they are all supposed to be comrades first.
Well, it could be that Sakura makes a really bad decision during a mission, like endangering the whole team because she was being too overconfident. Naruto, who has grown tired of her reckless behavior, tells her off. He might say things like 'Sakura, you can't keep acting this way. You're putting all of us at risk!'
Perhaps Sasuke has been ignoring Sakura's feelings for a long time in the fanfiction. Sakura decides she's had enough of his cold attitude. She tells him off for not being more considerate of her emotions. She might bring up all the times she's been there for him, and how he just brushes her off, and she demands some respect.
Sakura's obsession with Sasuke. Naruto might be fed up with how Sakura always puts Sasuke first, ignoring the people around her who truly care about her, like Naruto himself. So he tells her off to make her realize her wrong priorities.
One possible plot twist could be that they run off to a hidden village that is not known to the main ninja world. There, they discover a new type of chakra that has the power to heal any wound and bring back the dead. But this new power also has a dark side that attracts the wrong kind of attention from other ninja clans.
In this case, Sakura could be seen as the antagonist, at least in that particular moment when Naruto tells her off. Since Naruto is the one doing the telling - off, it's because of something Sakura has done or said that has pushed him to that point.
In this kind of fanfiction, Naruto might be really frustrated with his friends or the villagers. Maybe he's tired of being misunderstood or underestimated. He could start by calling out their false assumptions about him. For example, if people keep thinking he's just a troublemaker because of the Nine - Tails inside him, he would angrily tell them how he's been trying hard to protect the village all along.
Well, perhaps Harry is frustrated with Dumbledore's secrets. Maybe he's angry about being kept in the dark regarding his own fate or the danger around Hogwarts. So he decides to confront Dumbledore and let out all his pent - up feelings.
It could also be that Superman underestimates Danny's powers. Danny has his own set of unique abilities as a half - ghost. When Superman dismisses him or tries to boss him around, Danny stands up for himself and tells Superman off. He might show Superman some of his cool ghost powers like intangibility and invisibility in a rather sassy way to prove that he's not someone to be pushed around.
Perhaps Kate did something that went against the team's protocol. For example, she went solo on a lead without informing the team. Tony, who values teamwork highly, tells her off. He would likely remind her of how important it is to work together and follow the rules in NCIS.
One possible plot could be that Hermione tells off Molly for being overprotective of Ron. Maybe Molly is constantly interfering in Ron and Hermione's relationship, and Hermione finally has enough and gives her a piece of her mind about respecting their privacy and decisions as adults.