Well, Ron might be initially terrified of Slenderman. But when his friends are in danger, he would step up. The Weasley twins could try to play pranks on Slenderman, not fully realizing the danger at first. Dumbledore, on the other hand, would be very cautious. He might observe Slenderman from a distance, trying to understand his nature before taking any action. In a 'Harry Potter and Slenderman fanfic', these interactions would create a lot of interesting scenarios.
In a fanfic, Hagrid could be one of the first to encounter Slenderman in the Forbidden Forest. Hagrid's knowledge of strange creatures might make him think he can handle Slenderman, but he quickly realizes this is a different kind of threat. Meanwhile, the students at Hogwarts might start having nightmares about Slenderman, and Harry and his friends have to figure out a way to stop these nightmares and deal with Slenderman. Neville could also play a role, perhaps using his knowledge of plants to try and create some sort of defense against Slenderman.
The characters would interact in very strange ways. The Hogwarts students would be shocked by the Jackass crew's behavior. The Gryffindors might be initially intrigued, while the Slytherins would be disdainful. Harry and his friends would probably try to keep the Jackass guys out of trouble, but it would be difficult as they are always looking for the next wild thing to do.
Harry might be in awe of Elsa's powers at first. He would likely approach her with caution and respect, as he does with most powerful beings. Elsa, on the other hand, would be curious about Harry's world of wands and spells. They could start by sharing their stories. Harry would talk about Voldemort and his battles, and Elsa would share her experiences with her frozen kingdom and her isolation.
The Stargate characters are used to dealing with advanced technology and alien cultures. When they meet the Harry Potter characters, they might be initially confused by magic. For example, Jack O'Neill might be both amused and intrigued by the spells the wizards cast. Meanwhile, Harry Potter and his friends would be curious about the Stargate technology and the different worlds it can access.
The Avengers could be impressed by the magic of the Harry Potter characters. For instance, Captain America might be in awe of the Patronus charm. And the Harry Potter characters could be amazed by the Avengers' superpowers. Hermione might be very interested in Bruce Banner's scientific knowledge.
They could be classmates. For example, if the OC is in Gryffindor, they can study with Harry, Ron, and Hermione, participate in Quidditch practice together, and get into adventures in the Forbidden Forest. Or they could be rivals, like an OC from Slytherin who always tries to outdo Harry in every magical test or duel.
Well, in a Harry Potter fanfic involving Mummy Holmes, perhaps Mummy Holmes could be a mysterious figure at Hogwarts. She might use her detective - like skills to solve some of the strange mysteries that occur within the school. For example, if there are some strange disappearances or unexplained magical happenings, she could be the one to piece together the clues.
River Tam could be brought into the Harry Potter world as a sort of consultant. Given her knowledge and skills, perhaps Dumbledore summons her to help deal with a new and dangerous threat that has elements of both magic and advanced technology (like in her own world). She would then work closely with Harry and the others, sharing her insights and learning about magic in the process.
The interaction could be initially very tense. The Prime Minister might be skeptical and afraid of Harry Potter and the magic he represents. But as the story progresses, he would start to see Harry as an ally. For example, Harry might have to perform some magic in front of the Prime Minister to prove the existence of the wizarding world, and the Prime Minister would be both amazed and a bit intimidated. Later, they would communicate more freely, with Harry explaining the ways of the wizarding world and the Prime Minister sharing his concerns about how magic can impact the Muggle society.
Some characters might be lured into the 'dark land' out of curiosity. They may think they can find power or knowledge there. Others might be forced to enter it against their will, perhaps by dark wizards who want to use them for some dark purpose. Once inside, they could either be overcome by the dark forces or find a way to resist and fight back.
Hikaru is very observant, which could be useful in the wizarding world. He might notice small details about the magic around Hogwarts that Harry has overlooked. In return, Harry could teach Hikaru more about the serious aspects of magic, like defense against the dark arts. Their interactions would be a mix of cultural exchange, with Hikaru sharing his Japanese school experiences and Harry introducing the wizarding world traditions, and also a combination of their different personalities clashing and then meshing together as they face various challenges at Hogwarts.