Maybe a story about a human who accidentally stumbles into the hidden realm of Legendary Pokemon. There, they form a friendship with a Legendary that is an outcast among its kind. Together, they try to change the perception of the outcast Legendary in its own community while also finding a way for the human to return home.
A plot could involve a human accidentally stumbling into the hidden realm of Legendary Pokemon. There, they have to prove themselves worthy to the Pokemon and in the process, they develop a unique friendship. The human then uses this new - found relationship to bring peace between humans and Legendary Pokemon who have long been in conflict due to misunderstandings.
Mewtwo is also very popular. Given its origin as a genetically - engineered Pokemon, a human could interact with it in many ways. Maybe a human scientist wants to study Mewtwo further, or a young trainer tries to befriend Mewtwo and help it come to terms with its existence and powers.
Another possibility is Ash becoming Lugia. Lugia is known as the guardian of the seas. As Lugia, Ash could explore the deep oceans, discover new species of Pokemon, and safeguard the marine ecosystem. He could use his psychic and flying abilities to interact with both water - and flying - type Pokemon, bringing a new era of harmony between different Pokemon types.
Maybe the human is a huge Pokemon fan in our world. One day, they touch a strange, glowing Pokeball they found. Suddenly, they start to transform into a Charizard. Their new form gives them the ability to fly and use fire - type moves. They then decide to use their new powers to protect the Pokemon in the forest from poachers.
One common theme is friendship. Humans and Legendary Pokemon often form deep bonds of friendship in these stories. Another theme is adventure, as they usually go on exciting quests together.
Well, start with a strong concept. Imagine a world where Legendary Pokemon are hunted, and a small group of humans are trying to protect them. One of the humans forms a special bond with a Legendary, say Zekrom. The key is to build the relationship gradually. Show how the human and Zekrom learn to communicate, maybe through shared experiences or a common goal. Also, don't forget about the setting. Describe the environments where they interact, like a hidden cave filled with ancient Pokemon markings that holds a secret related to the Legendary Pokemon's power. This will make the fanfiction more immersive.
There are also original characters created by fans. These characters are often designed to have a unique backstory that ties them to Legendary Pokemon. For example, a character who was orphaned by a battle between two Legendary Pokemon and now has a vendetta against one of them, or a character who has a prophetic dream about a Legendary Pokemon and goes on a journey to find it.
One idea could be that Ash discovers a hidden power within himself that allows him to communicate directly with Legendary Pokemon. This power sets him on a journey to understand the secrets of the Legendary Pokemon world. He might start by having an encounter with Mew, who guides him to different places where he can learn more about his new ability.
One idea could be that Naruto discovers a strange portal that transports him to the Pokemon world. Since he has his ninja skills, he quickly adapts. He might start by befriending Pikachu, as it's one of the most iconic Pokemon. His goal could be to become a Pokemon master, using his determination from being a ninja.
One interesting storyline could be a human with a special bond to a Legendary Pokemon. For example, a trainer who discovers they have an ancient connection to Mewtwo. They might go on a journey to understand this connection and in the process, face challenges from other trainers or organizations who want to use this connection for their own gain.