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.
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.
Another idea is a crossover of the Upside Down and the Weirdness of Gravity Falls. Imagine Eleven using her powers in Gravity Falls and discovering new and strange things about the town. The kids might explore areas where the laws of physics seem to be a mix of both their worlds. For example, they could find a portal that leads to a place that has elements of both Hawkins and Gravity Falls.
One great Stranger Things crossover fanfiction is the one that combines it with 'Harry Potter'. It imagines the characters from Stranger Things attending Hogwarts. The way the Upside Down is incorporated into the magical world is really interesting. It adds a new layer of mystery and danger.
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.
A cool plot could involve the Demogorgon from Stranger Things being somehow related to It. The Losers' Club and the kids from Stranger Things have to figure out this connection and stop both threats. The Losers' Club could share their knowledge of facing supernatural horrors with the Stranger Things kids, and vice versa. For example, Dustin's knowledge of science might help in understanding the nature of It, while Bill's leadership skills could inspire Mike Wheeler.
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 great 'Stranger Things' crossover Steve - centric fanfiction is 'Steve in Another World'. It imagines Steve getting transported to a different dimension filled with characters from other shows. He has to use his wits and charm to survive and find his way back home. It's really exciting as it shows a different side of Steve.
One idea could be a story where a werewolf from the werewolf lore gets caught in the middle of a Supernatural's Winchester brothers' hunt. Maybe the werewolf has information about a powerful demon they are chasing.
In a world where Hawkins and the kaiju realm collide, Godzilla emerges in the Upside Down. Eleven senses his presence and is both terrified and intrigued. The Demogorgons try to attack Godzilla, but his atomic breath incinerates them. Meanwhile, the kids of Hawkins are in awe as they watch this giant creature fight their otherworldly enemies. Godzilla becomes an unexpected ally in their fight against the darkness of the Upside Down.