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.
A really interesting plot would be if multiple characters time travel at different points in the story. They end up in different time periods within the Stranger Things universe. Some could be in the 1950s when the first experiments related to the Upside Down might have started. And they all have to work together across time to finally close the Upside Down for good, even though they are in different eras.
Another plot could involve time travel to the future. Maybe a character gets sent forward in time and sees a desolate Hawkins, completely taken over by the Upside Down. They then have to figure out how to change the present to avoid that future. They might bring back knowledge of new powers or technology from the future to fight the monsters.
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 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.
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.
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 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.