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 the fanfiction, if Merlin goes to Hogwarts, he could become a sort of legend among the students. He'd probably be sorted into Ravenclaw because of his great wisdom. He might also introduce a new style of magic study that combines the magic of his time with the modern magic taught at Hogwarts. For example, he could teach about the use of natural elements in a more profound way than what the current professors do. This could lead to a new era of magic exploration at Hogwarts.
If Merlin goes to Hogwarts in fanfiction, it would be chaotic in a good way. He'd have a hard time adjusting to the modern rules. For instance, he might not understand why there are restrictions on using magic outside of classes. His magic is so instinctive and powerful. He could end up in a lot of trouble with the teachers at first, but then they would realize his value and start to learn from him. He could also have an interesting relationship with the house - elves, as he might have a different perspective on their servitude compared to the wizards at Hogwarts.
In this fanfiction, Dudley might face a lot of challenges. He's used to a different world than Hogwarts. He could struggle with magic classes as he has no prior knowledge. Maybe he'd be bullied at first for being a Muggle - born in the eyes of some students. Also, his relationship with Harry could change in a new and interesting way.
Well, in this fanfiction, Aang might bring his airbending skills to Hogwarts. He could use his bending to create interesting effects during Quidditch matches, like using air currents to speed up the broomsticks. He may also interact with the Hogwarts students in unique ways, perhaps teaching them a bit about his culture from the Avatar world.
If Fairy Tail goes to Hogwarts in fanfiction, it would be an interesting clash of cultures. The Fairy Tail guild is very rowdy and family - like. At Hogwarts, there are strict rules. The Fairy Tail members might get into trouble for their unruly behavior. For example, Gray might accidentally freeze part of the Great Hall during a fight with Natsu. And the Hogwarts students would be both shocked and intrigued by the Fairy Tail mages' powerful and flashy magic.
In the 'Percy goes to Hogwarts alone fanfiction', it's possible that Percy will discover new aspects of himself. Since he's alone, he has to rely solely on his own wits and skills. He could stumble upon some hidden secrets of Hogwarts that others haven't found. Also, he may have some unique encounters with the Hogwarts professors who take notice of his solitary situation and offer him special guidance or tasks.
Well, he might introduce some unique ninja skills to Hogwarts. Maybe he'd show the students how to move stealthily like in the ANBU, which could be a new concept for them. And his combat skills could be a shock to the wizards used to magic duels.
If Young Justice goes to Hogwarts in a fanfiction, they might form new alliances. For example, Superboy could team up with some of the stronger wizards like Hagrid. They could share their different kinds of strength. Superboy's physical strength combined with Hagrid's knowledge of magical creatures could lead to some really interesting adventures. And the wizards at Hogwarts would be both amazed and a bit scared of these new superhero visitors.
Well, perhaps in 'harry goes to hogwarts later fanfiction', he could form new friendships. He might meet students from different backgrounds or even different magical cultures, which would bring a whole new dynamic to his Hogwarts experience. He could also discover new magical abilities or spells that were previously unknown to him, which would help him in his adventures at Hogwarts.
She might have more time to explore the library on her own. Without the distraction of other students at first, she could discover some really rare and powerful magic books. This could give her a head start in her studies and she might even come across some ancient spells that could be useful later in her adventures at Hogwarts.
Well, he might discover some unique magical creatures in Canada that are different from those in the UK. Maybe he meets a friendly Canadian wizard who teaches him a special spell.
Hermione could also get some extra study time in the library. With no other students around, she would have access to all the books she wants without any distractions. Maybe she would find some really rare and powerful spells or magical knowledge that would help her throughout her time at Hogwarts.