Another possibility is a crossover with 'The Chronicles of Narnia'. The characters from 'The Magicians' could find themselves in Narnia and have to deal with the magical creatures and situations there. They might learn new magic from the Narnians or teach them some of their own.
A crossover with 'Doctor Who' would also be cool. The magicians could get caught up in one of the Doctor's time - traveling adventures. Maybe they help the Doctor solve a magical mystery that's affecting different time periods.
One idea could be exploring an alternate universe where the main characters have different magical abilities. For example, Quentin could have the power to control time instead of his usual skills. Another idea is to focus on a relationship that was only hinted at in the original, like a deeper connection between Eliot and Margo during their time at Brakebills. You could also create a fanfiction where the characters have to face a new, never - before - seen magical threat that forces them to use their powers in unexpected ways.
One idea could be that the characters from 'The Magicians' end up at Hogwarts for a magical exchange program. They'd have to adapt to the different magical rules and culture there. For example, Quentin might be fascinated by the concept of houses at Hogwarts, while Eliot could get into trouble for trying to introduce some of his more 'adult' magic tricks from Brakebills.
One idea could be that Quentin Coldwater from 'The Magicians' accidentally travels to the world of Harry Potter. He could be mistaken for a new exchange student at Hogwarts. He might try to use his Brakebills knowledge, like his unique spellcasting techniques, which would both amaze and confuse the Hogwarts students.
One idea could be a story where the characters from 'Harry Potter' and 'The Magicians' universes somehow cross paths. Maybe the Brakebills students end up at Hogwarts for a magical exchange program. They could learn different types of magic from each other and have some friendly (or not so friendly) competitions.
One interesting idea could be a crossover with "Moana". Elsa's ice powers and Moana's connection to the ocean could create a really cool dynamic. Maybe Elsa accidentally freezes part of Moana's ocean and they have to work together to fix it. Another could be with "Tangled". Rapunzel's long hair and her adventurous spirit combined with Elsa and Anna's journey in Frozen 2 would be exciting. Rapunzel could use her hair to help them climb mountains or cross chasms. A third idea is a crossover with "Beauty and the Beast". Belle's love for books and knowledge could help Elsa understand more about her powers and the history of magic in the Frozen world.
Quentin Coldwater from 'The Magicians' could be an interesting crossover character. He could visit Hogwarts and share his knowledge of magic from Brakebills. His more cynical view of magic compared to the generally more positive view in 'Harry Potter' could create some interesting dynamics. He might question some of the Hogwarts traditions and magical theories.
Well, a plot could involve a dark magician who has a secret connection to Harry's family. Maybe this dark magician was wronged by the Potters long ago and now wants revenge. This could lead to a complex story where Harry has to uncover the truth about his family's past while also dealing with the dark magician's schemes. The dark magician might also try to manipulate the wizarding world's politics to gain more power and make it easier to target Harry.
A Rambo crossover fanfiction could involve him crossing over into the Star Wars universe. Rambo's guerilla warfare tactics could be interesting in a galaxy far, far away. Maybe he helps the Rebel Alliance in a ground - based battle against the Empire. Or a crossover with the Predator series. Rambo would be a worthy opponent for the Predator, and the fight between them in the jungle could be epic. Also, a Rambo and John Wick crossover would be intense, two highly skilled fighters on the same side or perhaps initially at odds.
One idea could be a crossover with a modern military setting. The Decepticons could be trying to steal advanced military technology. Another is a crossover with a post - apocalyptic world where they are competing with other factions for resources. A third could be a crossover with a magical realm, with the Decepticons trying to harness the magic for their own evil purposes.
One idea could be a crossover with Naruto. Kakuzu could be sent to the Naruto universe and form an unlikely alliance with a shinobi village to fight a common enemy, like the Akatsuki from his own world turned rogue in the new universe. This would create a lot of interesting character dynamics and battles.