Sasuke could be one. Given their complex relationship, there could be a situation where Sasuke's newfound power or different ideology leads him to fight and beat Naruto in a fan - made story.
In some fanfictions, Naruto might get beat up when facing a powerful new villain who has some unknown jutsu. This could be a villain created by the fanfic writer to test Naruto's limits.
Naruto might be shocked at first. He has always seen Sakura as a friend, so being beaten up by her would be unexpected. He could try to talk to her and figure out what's wrong.
One possibility is that there's a misunderstanding between Naruto and Karui. Maybe Karui thinks Naruto has something to do with Sasuke's actions that have affected her village. So, in her anger, she attacks Naruto.
Well, in a lot of these fanfictions, Stiles can get beat up due to his own recklessness. He might be so determined to solve a mystery or help his friends that he doesn't think about the consequences and ends up in a dangerous situation where he is outnumbered and beaten up. Also, there could be miscommunication. Maybe someone thinks he has done something wrong when he actually hasn't, and they take matters into their own hands and beat him up.
Maybe some new kids in the neighborhood who are trying to establish their dominance and pick on Lincoln as an easy target. They might not know much about him or his family, but just see him as someone they can bully.
It could be an antagonist character in the Septiplier fanfiction world. Maybe someone who has a grudge against Jack for some reason, like a rival in a game or a love interest situation gone sour.
Well, in a 'jack gets beat up septiplier fanfiction', it could be that the writer is exploring a darker side of the relationship dynamics within the Septiplier fandom. It could be that Jack has made an enemy within the story's context, perhaps someone jealous of his relationship with another character. The beat - up could be a way to create drama and tension in the fan - created narrative.
One possible reason could be that he's gotten on the wrong side of a powerful client. Maybe he uncovered some shady business deals of the client during a case, and the client decided to take revenge by having him beaten up. Another reason might be related to his past. If some of his old associates from his pre - Harvey days found out he was now working in a high - profile law firm, they could be jealous and target him.
Orochimaru could also be a character to beat up Naruto in a fanfic. He's always been interested in the power of the Nine - Tails and Naruto as a whole. He might see an opportunity to capture Naruto and in the process, end up beating him up to subdue him.