Well, in some fanfictions, Eleven could be sent to Hogwarts by mistake. Since she has telekinetic powers, the wizards might initially be very confused and intrigued by her abilities. She could form an unlikely friendship with Hermione, as both are very intelligent and resourceful. And then there's the idea of the Demogorgons being related to some dark magic creature from the Harry Potter universe, and the two sets of characters have to figure out how to defeat them together.
One idea could be that the Demogorgon from Stranger Things somehow finds its way into the wizarding world of Harry Potter. The Hogwarts students would be both terrified and intrigued. They might initially try to use magic spells like Stupefy or Protego to fend it off. Hermione would probably be the first to start researching about this strange new creature, while Harry and Ron would be more on the front lines, trying to keep their classmates safe. The Stranger Things kids could also be magically transported to Hogwarts. Dustin would be amazed by all the magical creatures and would want to study them, Eleven might use her powers in new and unexpected ways in the wizarding world, like trying to communicate with magical portraits or using her telekinesis to move magic wands.
One idea could be that the Demogorgon from Stranger Things somehow enters the wizarding world of Harry Potter. The students at Hogwarts have to use their magic to fight it. Another idea is that Eleven has some sort of magical connection to the wizarding world and she gets a letter from Hogwarts. Maybe she could learn magic there and use her powers in combination with spells.
One idea could be that the Demogorgon from Stranger Things somehow finds its way into the wizarding world of Harry Potter. The Hogwarts students, with their magic, have to figure out how to stop it. Another could be that Eleven has some latent wizarding abilities and she gets a letter from Hogwarts, then has to navigate both the Upside Down and the magical world.
The Stranger Things characters would bring their own unique personalities and skills. For example, Joyce could be like a Muggle mother figure who somehow gets involved in the wizarding world and helps out with her no - nonsense attitude. Hopper could be an Auror - like figure, using his investigative skills to track down dark wizards or creatures that have a connection to the Upside Down. The kids would have to adapt to the rules of Hogwarts, but also introduce their own ideas. They might not be used to using wands, but they could find ways to incorporate their own abilities with magic, like using Eleven's powers to enhance a spell or something.
Steve and Ron would be a really fun pairing. Ron could introduce Steve to the wizarding world's food and traditions, and Steve could show Ron how to be a bit cooler and more confident in social situations. Also, Joyce and Molly Weasley would be an interesting pair. They are both strong mother figures, and it would be fascinating to see them bond over protecting their kids and dealing with the strange situations in both universes.
One interesting crossover could be the Demogorgon from Stranger Things appearing in the Forbidden Forest at Hogwarts. The students would be both terrified and intrigued by this strange, otherworldly creature.
Definitely Harry and Eleven. Harry has his own special powers in the wizarding world, being able to speak Parseltongue and having the power of love that saved him from Voldemort. Eleven has her telekinesis and other abilities. They could learn from each other. Harry could teach Eleven about the different types of magic, and Eleven could show Harry how to use her powers in a more creative way. Another interesting pairing would be Hermione and Dustin. They're both super smart. Hermione could introduce Dustin to the vast library of Hogwarts and all the knowledge it holds, while Dustin could share his knowledge of science and technology with Hermione, and they could come up with some really cool ideas that combine magic and science.
Harry Potter himself could also be a very powerful character in the merged universe. He has his own magical skills and the experience of fighting dark wizards. He could teach the Stranger Things characters about the wizarding world and use his magic to protect them from the Upside Down threats. Also, his connection to Voldemort might give him some unique insights into defeating any dark forces that come from the merging of the two universes.
Well, time travel is a cool theme. Some fanfictions might have the characters going back in time to prevent certain events in the show from happening, like the opening of the gate to the Upside Down. Then there's the theme of alternate universes. Imagine a universe where Eleven was raised by Brenner in a completely different way. And of course, the theme of romance. You can find a lot of fanfictions about Mike and Eleven's relationship, but also other possible pairings among the characters.