A Stranger Things crossover fanfic with the Avengers would be really exciting. The Hawkins kids could team up with the Avengers to fight off the Demogorgon which has somehow become a threat to the entire world. Or a crossover with Doctor Who. The Doctor could land in Hawkins and help the gang deal with the Upside Down, bringing his unique time - traveling and alien - fighting skills. Also, a crossover with the Ghostbusters would be fun. They could come to Hawkins to deal with the strange entities from the Upside Down using their proton packs.
Perhaps a story where Steve and Robin get involved in a time - travel adventure within the Stranger Things universe. They accidentally stumble upon a portal that sends them back in time, and they have to find their way back while also changing some small but significant events in the past.
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.
Another great concept is a crossover where Eleven's powers are mistaken for some kind of witchcraft by the Supernatural universe. Sam and Dean initially think she's a threat but then realize she's actually a powerful ally against a common enemy. They could be facing a powerful being that can manipulate time and space, and Eleven's telekinetic and telepathic abilities are crucial in defeating it.
Perhaps in a fanfiction, Hopper is hurt while protecting Eleven. The government agents are after her again, and Hopper steps in between them. They use some sort of experimental weapon on him that causes him internal injuries. Eleven then has to use her powers to keep him alive while they try to find a way to heal him. This could lead to a lot of emotional moments between them as Eleven blames herself for Hopper getting hurt.
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.