In some fanfictions, Naruto has to beat up members of a secret organization plotting against Konoha. These could be powerful individuals with dark motives, and Naruto would use his determination and ninja skills to take them on.
Often, it's other powerful ninjas from rival villages. For example, shinobi from the Hidden Mist Village might be opponents. They could have unique abilities that Naruto has to overcome to beat them up.
One 'naruto beat up' fanfiction could be about Naruto facing a powerful new enemy in a hidden village and having to fight his way out. Maybe he uses new jutsu combinations to beat up the bad guys.
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.
Often, it's powerful villains like Orochimaru who make a comeback in fanfiction. Naruto has to use his growth and new abilities to defeat him once and for all.
Another could be Kaguya Otsutsuki. However, in the fanfiction, she may return in a more powerful form. Maybe she has found a way to break free from the seals completely and has amassed even more power from other dimensions. Naruto, being abnormally strong, would still have a tough time fighting her as she is one of the most powerful characters in the Naruto lore.
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.
One common reason could be for character development. By having Steve Rogers get beat up, it shows his vulnerability. It can also be a plot device to introduce a new character who saves him, creating a new relationship or alliance. Another reason might be to explore the limits of his physical and mental strength. In some fanfictions, the writer may want to deviate from the typical 'invincible' image of Steve Rogers to make the story more interesting and unpredictable.
One common opponent could be other dark wizards. These wizards might also be skilled in dragon dueling and use their dragons to spread chaos. They would see Harry as a threat and try to defeat him in duels.
Probably not. Most fans love to see Spider - Man in his heroic element, swinging around and saving the day. Fanfictions that involve him getting beaten up go against the typical image of him being a strong and capable hero. So, it's less likely that there are popular ones with this theme.
Sasuke would still be a likely opponent. Even though Naruto has enhanced speed, Sasuke has his own unique abilities like the Sharingan which can predict movements to some extent. Their battles would be even more intense, with Naruto's speed against Sasuke's strategic use of his Sharingan and ninjutsu.
Often, he overcomes powerful villains like Madara Uchiha. In some fanfictions, Madara might be even more powerful than in the original, but Naruto's newfound abilities allow him to defeat him. Another could be Orochimaru, who might have some new and dangerous experiments, but Naruto is still able to stop him.