One idea could be a crossover with Harry Potter. Imagine the wizards from Wizard101 going to Hogwarts and interacting with the students there. They could compare their different magic systems, like how spells are cast in Wizard101 compared to using wands in Harry Potter.
Another interesting idea is a story where a RWBY character discovers they have latent wizard powers. Ruby, for instance, could find out she has wizard abilities on top of her scythe skills. This could lead to a new arc in her character development as she tries to balance and master both her combat skills and newfound magic. It would also open up new storylines with her team and the other characters.
One idea could be that the dark wizard tries to steal the magic of Equestria in the MLP world. He might be lured by the pure and powerful magic there.
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.
One idea could be a crossover where characters from Harry Potter and Wizard 101 team up to fight a new dark force. For example, Harry Potter and his friends could meet up with the wizard from Wizard 101 in a magical realm that is a blend of both universes.
One idea could be a prequel where we see how the characters were before Dorothy arrived in Oz. For example, exploring the Scarecrow's life in the field, alone and longing for a brain. Another could be a story where Dorothy returns to Oz as an adult and has to deal with new threats. Or perhaps a fanfiction where the Wicked Witch of the West was actually misunderstood and we see her side of the story.
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.
Another option is a crossover with 'The Little Mermaid'. The characters from the descendants could meet Ariel and her friends underwater. Evie might be interested in the mermaid fashion and could design new outfits inspired by the underwater world.
Another possibility is a crossover with the world of Naruto. The characters from MHA could participate in the Chunin Exams. Bakugo might be really into showing off his skills in this new and different kind of competition.