One idea could be Ash encountering a Legendary Pokemon that has lost its memory. Ash then tries to help it regain its memories while protecting it from poachers. Another could be a time - travel plot where Ash is sent back in time and has to team up with a Legendary Pokemon to prevent a great disaster from happening in the past that would change the future of the Pokemon world.
Well, a plot could involve Ash being chosen by a Legendary Pokemon to be its emissary. He has to go on a quest to unite all the Legendary Pokemon to stop an evil force that is trying to disrupt the balance of the Pokemon universe. Or perhaps Ash stumbles upon a hidden realm where Legendary Pokemon live, and he has to prove himself worthy to stay and learn their secrets.
A really cool plot could be that Ash gets turned into a Legendary Pokemon himself for a short period. He has to figure out how to control his new powers and use them to save his friends who are in danger from a powerful villain. Also, a story where Ash discovers a Legendary Pokemon egg and has to protect it from all kinds of threats until it hatches, and then raise the newly - hatched Legendary Pokemon.
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.
Perhaps Ash gets a special ability from a Legendary Pokémon, like the ability to communicate telepathically with all Pokémon. This causes him to be hunted by an organization that wants to use this power for their own gain. With the help of various Legendary Pokémon that he befriends on his journey, Ash has to protect himself and his new power.
A third plot idea might be that Ash is the chosen one to bring peace between the different factions of legendary Pokemon. There could be some legendary Pokemon that are at odds with each other, and Ash, with his unique bond with Pokemon in general, is the only one who can bridge the gap. He would have to understand the reasons for their conflicts, which could be rooted in ancient history or disputes over territory. Along the way, he'd have to convince both sides that cooperation is better than conflict, and he might use his own experiences with his Pikachu and other Pokemon friends to show them how different species can get along.
In a fanfiction like this, the plot could revolve around the fact that the egg is the key to restoring balance in the Pokemon world. The legendary starter that hatches from it is initially weak but grows rapidly under Ash's care. However, there are forces at work that want to use the starter for their own evil gains. Ash has to team up with his friends and other Pokemon to fend off these threats. The story could also explore the relationship between Ash and the legendary starter as it grows, how they learn from each other, and how their bond becomes unbreakable.
Another interesting plot could be a time - travel story where Ash and Cynthia are sent back in time to the early days of the Pokémon world. They have to adapt to the different rules and ways of training Pokémon back then while also trying to find a way back to their own time.
One plot idea could be that Ash is captured by a new and powerful villain who wants to use his Pokémon knowledge for evil. Another could be that he's captured by a group who mistake him for someone else and he has to prove his identity while trying to escape. Maybe he's captured by an ancient Pokémon that has some sort of grudge against trainers and he has to convince it that not all trainers are bad.
Well, a great plot might be that Ash enters a special tournament where the grand prize is a chance to catch a Pseudo - Legendary Pokemon. He has to battle tough trainers along the way. Also, it could be that a Pseudo - Legendary Pokemon has some sort of unique ability that Ash discovers he needs to help solve a big problem in the Pokemon world. Maybe it can control a powerful energy source, and Ash must earn its trust to use that power for good.
Well, there's a fanfic called 'Ash's Legendary Encounter'. In it, Ash travels to a far - off region filled with ancient ruins. There, he discovers a legendary pokemon that is the guardian of the ruins. With his unwavering determination and the help of his friends, he manages to catch it. Also, 'Legendary Bonds' is a good one. Ash forms an unexpected bond with a legendary pokemon through a series of events that lead to its capture.
Well, 'Ash and the Legendary Guardians' is quite popular. It shows Ash being chosen by a group of Legendary Pokemon as their protector. Then there's 'Legendary Encounters: Ash's Story' where he has unexpected and exciting meetings with different Legendary Pokemon throughout his journey. And don't forget about 'Ash's Legendary Alliance' which involves Ash making alliances with Legendary Pokemon to compete in a special tournament.
Another plot could involve a Pokémon contest where May is a contestant and Ash is there to support her. But something goes wrong with the contest rules due to a villain's interference. Ash and May then team up to expose the villain and make sure the contest is fair for all the contestants. It could also be a chance for their relationship to grow deeper as they face challenges together.