A fsn crossover with the world of Attack on Titan could be really exciting. For example, the Servants could help in the fight against the Titans. Saber's combat skills would be a great asset. Or a crossover with a more modern - day superhero universe. Imagine Shirou with superpowers and how he would use them. You could also have a crossover with a post - apocalyptic game like Fallout, where the characters have to survive in a desolate world while still dealing with their own Grail War - like conflicts.
The 'FSN and Naruto Crossover' fanfic is also worth checking out. It imagines how the Servants from FSN would fare in the world of Naruto, with its own ninja battles and power systems. There are some really cool fight scenes and unexpected alliances.
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.
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.
One idea could be a Venom crossover with Spider - Man in an alternate universe where they have to team up against a common enemy like Carnage. Another could be a Venom and Deadpool crossover, with their crazy personalities clashing and then somehow working together. Maybe a Venom crossover with the X - Men, where Venom's alien nature causes some interesting reactions among the mutants.
For a Monokuma crossover fanfiction, consider a crossover with 'Final Fantasy'. Monokuma could be a summoned monster by an evil villain in the game's world, causing all sorts of problems for the heroes. He might even try to take over the crystals. A crossover with 'Persona' could also be interesting. Monokuma could enter the Metaverse and start his own 'Palace', recruiting Shadows to enforce his rules. And how about a crossover with 'One Piece'? Monokuma could be a new Warlord of the Sea, using his wacky yet dangerous devices on the Straw Hat Pirates and their allies.
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.