In a crossover with Bleach, Kakuzu could end up in the Soul Society. His immortality and combat skills would make him a formidable foe or a strange ally. He might be interested in the power structure there and try to manipulate it for his own gain, leading to epic battles against the Soul Reapers.
A crossover with One Piece might also be cool. Picture Kakuzu on the Grand Line, his unique abilities making him a feared pirate or a bounty hunter. He could get involved in the search for the One Piece, clashing with the Straw Hat Pirates at first but then realizing they share some common goals against the World Government's corruption.
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.
Maybe Kakuzu's bloodline has a hidden sealing technique. In the fanfiction, Naruto is trying to learn more about sealing, and Kakuzu, seeing potential in Naruto, decides to teach him this special bloodline - based sealing technique. However, the Akatsuki finds out and tries to stop them, thinking Naruto will use it against them. This leads to a series of battles and a race against time for Naruto to master the technique.
One idea could be that Sakura and Kakuzu are forced to team up on a mission in a strange land. They start off distrusting each other because of their different backgrounds. Sakura is from Konoha and values life, while Kakuzu is a money - hungry criminal. But as they face various challenges like dangerous beasts and traps, they gradually learn to rely on each other's skills. Sakura's medical ninjutsu helps when Kakuzu gets injured, and Kakuzu's combat experience protects Sakura from some tricky attacks.
Well, in fanfiction, one interesting aspect could be their unique relationship dynamic. Hidan's wild and impulsive nature contrasts with Kakuzu's more calculating and money - hungry side. This can lead to some really engaging character interactions, like when they are on missions together and have to deal with different situations based on their personalities.
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.
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.