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.
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.
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.
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.
The plot could involve a new, super - powerful entity that is a combination of the Upside Down's dark forces and a kaiju - like being. This entity starts to wreak havoc in both Godzilla's world and Hawkins. The Stranger Things kids find a way to call Godzilla for help, and Godzilla arrives just in time to face this new threat. As they fight together, they learn about each other's strengths and weaknesses. Eleven uses her powers to enhance Godzilla's attacks, while Godzilla's presence scares off some of the smaller Upside Down creatures, giving the kids a chance to regroup and plan their next move.
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 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.
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.
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 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.