A really cool concept could be that the kids from Stranger Things are actually Muggle - born witches and wizards. They just haven't received their Hogwarts letters yet because of some interference from the Upside Down. When they finally do get to Hogwarts, they introduce some of their unique skills and ways of thinking from their experiences in Hawkins. They could even form a new club at Hogwarts that combines their knowledge of the Upside Down and magic.
Well, perhaps the Mind Flayer from Stranger Things is actually an ancient dark magic entity in the Harry Potter universe. The characters from Stranger Things team up with Harry, Hermione, and Ron to defeat it. Or what if the Upside Down was a corrupted part of the wizarding world that was sealed away long ago, and now it's starting to seep back into the normal wizarding world, causing chaos.
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.
One idea could be that the Upside Down from Stranger Things somehow connects to the multiverse in Marvel. Maybe Eleven's powers could be used to traverse different Marvel universes, leading to some really cool crossovers like Eleven meeting Scarlet Witch and having a power - off or learning from each other. Another could be the Demogorgons invading New York City in the Marvel universe, and the Avengers having to team up with the kids from Stranger Things to fight them off.
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.
Well, one plot could be that Eleven discovers a new power that only Mike can help her control. It makes them have to spend a lot of time together in secret, away from the others. This creates a really close and intense bond between them as they figure out this new aspect of her abilities.
One idea could be that the Demogorgon from Stranger Things somehow ends up in Derry from It. The Losers' Club and the kids from Hawkins could team up to fight it. They would have to combine their different skills and knowledge of the supernatural.
One idea could be that the Losers' Club from 'It' somehow ends up in Hawkins, Indiana, during the events of 'Stranger Things'. They could team up with the kids from 'Stranger Things' to fight a new, combined threat that has elements of both the Upside Down monsters and Pennywise's terrors. For example, the Mind Flayer could merge with Pennywise in some way, creating an even more terrifying entity.
One idea could be a crossover where the ninjas from Naruto enter the Upside Down in Stranger Things. They use their jutsu to fight the strange creatures there. Another could be characters from Stranger Things learning chakra control in the Naruto universe. Maybe Eleven could use her powers in combination with Naruto's Rasengan.
One idea could be that Naruto characters are transported to the Upside Down in Stranger Things. They use their ninja skills to fight the strange creatures there. For example, Naruto could use his Rasengan against the Demogorgon.
You could have an OC that is a transfer student from a different town where strange things have also been happening. This OC could have some knowledge about the otherworldly events that the kids in Hawkins haven't encountered yet. Or make an OC who is a scientist's assistant, secretly helping the main characters by providing inside information from the lab. They might also be in danger because of this connection to the scientific side of the mystery.
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.