A great idea for a Pokemon Detective Pikachu fanfic could be to set it in a different region. For example, a region that is mostly covered in ice. Pikachu and Tim would have to adapt to the new environment and the different types of Pokemon there. They could be investigating a series of disappearances of ice - type Pokemon. Maybe there's a secret underground lair where a powerful but misunderstood Pokemon is hiding and Pikachu has to use his detective skills to uncover the truth and save the day.
A more complex plot could involve Detective Pikachu trying to figure out a conspiracy among a group of humans who are trying to use Pokemon for illegal and unethical experiments. This would involve Pikachu teaming up with other Pokemon and human allies, like Tim, to expose the bad guys. They would have to face many obstacles and dangerous Pokemon controlled by the villains along the way.
One idea could be to create an OC who is a detective from a different region, like Unova. They could have a unique Pokemon partner, like a Zorua that can transform and help with investigations in sneaky ways. Their backstory could involve coming to Ryme City to learn new detective techniques.
One idea could be that Pikachu wakes up in a strange part of the mystery dungeon with no memory of how it got there. It has to team up with other Pokemon it meets along the way, like a Charmander and a Squirtle, to figure out the mystery of the dungeon and also find a way to get back home.
A plot could revolve around Pikachu getting sick and Lillie using her knowledge of Pokémon care to nurse it back to health. During this time, they develop a deep friendship. Then, they notice some strange behavior in other Pokémon around them. Together, they start to investigate and find out that there is a pollution source affecting the Pokémon. Lillie and Pikachu then try to find a way to clean up the pollution and restore the health of the Pokémon in the area. This would show their caring and adventurous sides.
One idea could be that the female Pikachu has unique electric powers that no other Pikachu has. She could go on a journey to discover the origin of these powers, facing various challenges along the way, like battles with powerful Pokemon and difficult terrains.
One idea could be a story where Pikachu gets separated from the trainer in an unknown and dangerous area, like the mysterious depths of Viridian Forest. It has to find its way back while making new Pokemon friends and facing challenges from wild Pokemon.
Here's a plot idea. There's a new Pokemon trainer in town who only wants to catch Pikachu and Mimikyu. They keep running away from the trainer and end up in an old, abandoned Pokemon sanctuary. There, they find out about the history of the place and some secrets about their own species. They also meet some old and wise Pokemon who teach them new moves. Eventually, they outwit the trainer and decide to stay friends and protect the sanctuary from any harm in the future.
One idea could be that Pikachu suddenly loses its electric powers and Ash has to find a way to help it regain them. This could involve a journey to a mysterious place that is said to have the power to restore Pokemon abilities.
One idea could be an OC who is a detective trainee and teams up with Detective Pikachu to solve a mystery in a new part of the Pokemon world. Maybe they are investigating a series of disappearances of rare Pokemon. Another plot could involve an OC with a special ability to communicate with certain types of Pokemon that Pikachu can't, and together they use their combined skills to stop a villain who wants to use these Pokemon for evil.
In a 'pokemon red's pikachu fanfic', it might focus on Pikachu's growth. For example, how it becomes stronger not just in terms of battling but also in its understanding of the Pokemon world. It could also explore Pikachu's emotions, like how it feels when separated from Ash or other trainers in the fan - created scenarios.