A cool fanfiction concept might be that some of the Stranger Things kids are actually reincarnations of Naruto characters. For example, Mike could be Naruto in a different life. They start to remember their past lives and use their old skills. Or perhaps the Demogorgon is actually a result of some dark jutsu experiment gone wrong in a combined Naruto - Stranger Things world.
In a fanfiction, the Leaf Village could exist in parallel to Hawkins. And there could be a plot where the two worlds start to merge. The characters from both have to figure out how to stop it if it's a bad thing, or how to make the best of it if it's a good opportunity. The ninja skills and the telekinetic powers would create really interesting interactions.
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.
Another concept is to have the Upside Down merge with the Halloween spirit world. Characters from Stranger Things might find themselves in a strange hybrid world where normal Halloween monsters like vampires and werewolves co - exist with the Upside Down creatures. They would have to figure out how to survive and if there's a way to separate the two worlds again.
One idea could be that a character travels back in time to prevent the initial opening of the Upside Down. They could change a key event that led to all the chaos in Hawkins. For example, if they stop Eleven from using her powers in a certain way that originally weakened the barrier between dimensions.
One idea could be that the next generation discovers a new dimension that's even more dangerous than the Upside Down. They have to use the knowledge passed down from their parents, but also develop their own unique powers to deal with it.
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.
They could meet in a virtual reality created by some new government experiment. Mike gets trapped in this virtual world and Eleven, with her powers, is the only one who can sense where he is. She enters the virtual world to save him. Inside, they face all kinds of digital monsters and challenges. As they fight their way through, they also realize their feelings for each other are even stronger in this strange new environment.
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.
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.
One idea could be that the Upside Down in Stranger Things is somehow connected to a dark and distorted Pokemon world. The Demogorgon could be like a super - powerful and corrupted Pokemon. The kids from Stranger Things would have to team up with their favorite Pokemon to fight against the forces of the Upside Down.