You could write a fanfic where the characters swap their supernatural abilities for a short period. The vampire becomes a werewolf for a full moon cycle and vice versa. This would lead to a lot of humorous and also challenging situations as they adapt to their new 'bodies' and instincts.
One idea could be exploring what would happen if the main characters had to face a new supernatural threat that was a combination of different mythical creatures. For example, a werewolf - vampire hybrid that was immune to the usual ways of killing them.
A bumblebee human fanfic could involve a bumblebee that can talk to a human and they team up to save their shared environment. The bumblebee might tell the human about the importance of certain plants for its species and they work together to protect those plants from destruction.
One idea could be a story where a human trainer has a special bond with a Pokemon that allows them to communicate telepathically. For example, the trainer and their Pikachu can share thoughts during battles, giving them an edge. Another could be a human who gets lost in the Pokemon world and has to team up with various Pokemon to find their way back home. They learn about the different cultures and habitats of the Pokemon along the way.
One idea could be to explore how the different species' characteristics translate into human traits. For example, the herbivores' fear of predators might be like a human's fear of a more powerful or dangerous group. Another idea is to set the story in a human - like school environment where there are still social hierarchies based on similar principles as in Beastars. Maybe the story could focus on a human 'wolf' character who tries to break the cycle of aggression and prejudice.
One idea could be a dragon who is injured and is found by a human. The human nurses the dragon back to health and they form an unbreakable bond. Then they go on adventures together, facing various challenges in a magical world.
One idea could be a story where a human scientist is studying sharks and forms an unexpected bond with a particularly intelligent shark. They communicate in some unique way and end up helping each other, like the shark leading the human to a hidden underwater treasure which in turn helps the human protect the shark's habitat.
One idea could be a story where a human trainer has a unique bond with a Pokemon that allows them to communicate on a deeper level, and they go on adventures together. Another could be a world where humans and Pokemon co - exist in a more integrated society, with some Pokemons having jobs like in a human town. And perhaps a story about a human who discovers a rare Pokemon and has to protect it from those who would use it for wrong purposes.
A 'human x noivern fanfic' could be about a human who has the ability to communicate telepathically with Noivern. They team up to stop an evil force that is threatening the Noivern's natural habitat. Along the way, they learn about each other's cultures and the human gains a new perspective on the world of Pokémon, specifically the Noivern species.
One idea could be a story where the Chipmunks become human for a day and experience high school. They could struggle with typical teenage problems like fitting in and dealing with bullies.
One idea could be a story where a human trainer has a special bond with a Pikachu that allows them to communicate telepathically. Another could be a human getting lost in the Pokemon world and having to team up with various Pokemon to find their way back home. Or perhaps a story about a human who discovers they have the power to transform into a Pokemon.
One idea could be a story where a human hunter is saved by a Barioth during a particularly dangerous hunt. The Barioth, instead of attacking the human, senses a greater threat and defends the human. This event then forms an unlikely bond between them, and they start to communicate in a unique way, perhaps through body language and mutual respect for each other's hunting skills.