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.
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.
Merlin could be a guiding spirit for the founders. He might appear in their dreams, offering advice on how to create a school that would last for centuries and educate young wizards and witches. His ancient wisdom would be invaluable in making decisions about the school's rules, curriculum, and the sorting system.
Well, in fanfiction, Merlin as a teacher at Hogwarts could bring a whole new level of magic. He might teach some ancient and powerful spells that are long forgotten in the regular curriculum. His teaching style could be very different, perhaps more hands - on and based on his own experiences from his time. He could also have some really interesting interactions with the students, like sharing stories of his adventures which would both inspire and terrify them at the same time.
One possibility is that Merlin brings his ancient magic knowledge. He might introduce spells and magical concepts long forgotten in the Hogwarts curriculum. For example, he could teach students about pre - Hogwarts era potion - making techniques that are more in tune with nature.
You can start by looking on popular fanfiction websites like Archive of Our Own or Fanfiction.net. Just use the search terms 'Hogwarts' and 'Merlin' and you should get a list of relevant fanfics.
Well, Merlin could be a really powerful wizard at Hogwarts. He might be in Gryffindor because he's brave, like when he faces all those dangerous situations in Camelot. Arthur, on the other hand, would probably be a bit confused at first. But he'd soon adapt and be great at Defense Against the Dark Arts. He'd be the one always ready to fight off any threats to Hogwarts, just like he did in Camelot.