Madara Uchiha is another strong contender. He's extremely powerful and has his own motives that could put him at odds with Naruto. If they were to face off in a fanfic, Madara's strength and experience could potentially lead to him beating Naruto.
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.
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.
A former friend or teammate who has turned to the dark side could be an antagonist. For example, if someone like Sasuke had completely given in to the dark side and decided to attack Naruto for some reason. Or it could be a new and mysterious character who has a vendetta against the Uzumaki clan and sees Naruto as the prime target to take revenge on the clan. This character could be very powerful and use their abilities to beat Naruto up.
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.
I think Hinata and Rias could be paired up in some way. Hinata's gentle nature and her strong loyalty could complement Rias' leadership and determination. They might form an unexpected friendship or even a strategic alliance within the fanfiction's plot.
One popular scenario could be Naruto getting beaten up during a spar with Sasuke that gets out of hand. Sasuke, in his pursuit of power, might go a bit too far in his attacks on Naruto.
Well, obviously Naruto himself would be the main character. There could also be his friends like Sasuke and Sakura, who might be involved in the interactions between the two Narutos. Maybe they are confused at first by seeing two Narutos, but then they help them figure out what's going on.
Sasuke is often an interesting character in these fanfics. Since he's constantly seeking power, in a power - up fanfic, he might develop some really cool new abilities. He could gain the ability to control a new type of chakra, like some kind of dark chakra that gives him extra - powerful attacks.
Iruka and Ducky. Iruka is a teacher in Naruto, and Ducky is an experienced medical examiner in NCIS. They could bond over their love for knowledge and helping others. Iruka could learn about modern medical examination from Ducky, and Ducky could be fascinated by Iruka's understanding of ninja - based education and how it shapes young minds.
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 would likely be one of the main characters to interact with Naruto. Given their complex relationship, Sasuke might be either supportive or skeptical of Naruto's newfound power. Sakura could also be involved, perhaps trying to heal Naruto if the dark power has negative effects on him. Kakashi would surely step in as well, using his wisdom to guide Naruto and make sure he doesn't lose himself in the darkness.