Well, he might show off some really old - fashioned but extremely powerful magic that would shock everyone at Hogwarts. Maybe he'd befriend Harry Potter and teach him some ancient spells that could defeat Voldemort in a whole new way.
He might use his spider - powers to outperform in Quidditch. His agility could make him an excellent seeker, zipping through the air to catch the golden snitch much faster than others.
In a 'Merlin and Arthur at Hogwarts' fanfiction, the story could start with Merlin accidentally casting a spell that sends them both to Hogwarts. At Hogwarts, Arthur is amazed by the magical creatures and the Quidditch games. Merlin tries to blend in but his unique abilities make it difficult. They both get caught up in a mystery within the school, like a hidden chamber that holds a powerful and ancient magic. As they explore, they discover that their fates are somehow linked to the history of Hogwarts, and they must use their combined skills - Arthur's leadership and Merlin's magic - to solve the mystery and find a way back to their own world.
There is a fanfic titled 'Merlin's Influence at Hogwarts' which details how Merlin's magic and his legend start to affect the students and teachers at Hogwarts. It's really interesting as it combines the lore of both universes. In this story, the characters from Hogwarts start to discover ancient spells that are related to Merlin's time, and they have to figure out how to use or control them. This creates a lot of tension and adventure throughout the story.
Merlin would likely be seen as a bit of an enigma at Hogwarts. His magic is different from the norm, so he might be a bit of an outsider at first. But his kind heart and loyalty would soon win over his classmates. Arthur would be the brave and headstrong character, perhaps a bit like a Gryffindor version of his usual self. He'd be eager to learn about combat magic and would probably be a natural leader in group projects or Quidditch teams.
The plot might involve Naruto making friends with some of the Hogwarts students. He could show them his unique fighting style, like the Rasengan, and they in turn could teach him some basic spells. For example, Hermione could be intrigued by Naruto's chakra system and Naruto could be amazed by her knowledge of magic. There could also be a conflict where Naruto has to face some dark wizards using his ninja abilities, while also learning how to use magic in a pinch.
There could be a character like Tom, a Smeltings student who is very disciplined but curious about the magical world of Hogwarts. He might be the main character who drives the story forward as he experiences all the new things at Hogwarts.
In the 'sakura goes to hogwarts fanfiction', it could be that Sakura is a transfer student. She would face the challenges of adapting to the different magical system in Hogwarts. For example, she has to learn about potions and spells in the Hogwarts way. Along the way, she might discover a hidden mystery in Hogwarts that only she, with her special background, can solve. Maybe it has something to do with an ancient artifact that links her world to Hogwarts.
Well, it could be about Mal, who might be a new student at Hogwarts. Maybe Mal discovers a hidden magic ability that no one else has. He could make new friends and together they face some sort of mystery in the castle, like a secret room that holds an ancient magic that's in danger of being misused.