They could start off as enemies. For example, Billie might initially suspect Mrs. of being involved in something dark. But as they interact more, they find out they have a common goal of protecting a particular character from the forces of the Upside Down. This could lead to a story full of tension and character development.
One plot idea could be that Billie and Mrs. form an unlikely alliance to solve a mystery in Hawkins that the main group has overlooked. Maybe they discover a hidden connection between the Upside Down and a local family's history.
A more emotional plot could be that Billie has some sort of past trauma related to the events in Stranger Things. Mrs., being an empathetic figure, tries to help her overcome it. Along the way, they learn more about each other's strengths and weaknesses and become closer friends, all while the strange happenings in Hawkins continue in the background.
Billie could be developed by giving her some new abilities or skills. For instance, she could have an uncanny sense for detecting when the Upside Down is about to seep into the real world. Mrs. could be developed by showing her as a mentor figure. She could start teaching the younger characters in Hawkins some valuable life lessons that also tie into dealing with the supernatural threats.
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 to explore Billie Joe's backstory before the events in the song. Maybe create a fictional account of his childhood, his dreams, and how he got into the situation that led to the events in 'ode to billie joe'.
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.
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.