It could be about Ash finding a Gyarados in an unexpected place, like a hidden lake in a forest. Then he tries to train it and form a bond with it, facing challenges along the way.
You can try searching on popular fanfiction websites like FanFiction.net or Archive of Our Own. Just type in 'Ash Gets a Gyarados' in the search bar and see what comes up.
One possible plot could be that Riolu is being chased by some bad guys who want to use its power. Ash rescues Riolu and they team up. As they journey, Riolu learns from Ash's kindness and determination. They enter Pokémon battles together, and Riolu's unique fighting style combined with Ash's strategies help them win. Also, their relationship could grow deeper, with Riolu becoming one of Ash's closest Pokémon companions.
One possible plot could be that Ash hears of a powerful Pokémon tournament where the winner gets a special wish. He decides to enter with his current team, battles through tough opponents, and in the final, he faces a trainer with a Charizard. After winning, he uses his wish to get his old Charizard back.
In this fanfiction, it could be that Ash has a really good strategy. He might first make Gyarados use up a lot of its energy by making it attack Charmander's illusions or decoys. Then, when Gyarados is tired, Charmander can step in with its full power. Charmander could also use the environment to its advantage. Maybe there are some flammable objects around that can be set on fire by Charmander, creating a more powerful and widespread fire attack that Gyarados can't avoid.
A possible plot could be that Ash finds a Cleffa in a forest. Cleffa is injured, and Ash decides to take care of it. As Cleffa heals, it starts to show an interest in Ash's Pokemon training methods. Eventually, Cleffa joins Ash's team and helps him in battles with its unique moves.
In a different plot, Ash and his friends come across an injured Togepi. Ash nurses it back to health. Togepi then becomes attached to Ash. However, there is a prophecy that Togepi will one day have to make a great sacrifice to save the Pokémon world. Ash is determined to change this fate and goes on a quest to find a way to break the prophecy while having many adventures along the way.
Well, one possible plot could be that Ash and Bonnie have been on a long journey together and developed deep feelings over time. They might face challenges from their friends or society as they deal with the unexpected pregnancy. Ash could struggle with his responsibilities as a soon - to - be father, while Bonnie tries to figure out how to balance her own dreams with the new situation.
One possible plot could be that Ash and Lily are in a relationship that has been growing secretly. They face challenges from their friends and families when they find out about the pregnancy. Ash might struggle with the sudden responsibility but then decides to step up and be there for Lily and the baby.
In this fanfiction, it could be that Ash's marriage is a way to bring peace between different regions. He marries a princess or a powerful trainer from a rival region. Their union symbolizes the unity of Pokémon trainers everywhere. The story could then detail the political and social implications of their marriage, as well as the more personal aspects like their relationship building and how they learn to love and respect each other's Pokémon.
One possible plot could be that Ash wakes up one day as a girl and has to deal with all the changes in his daily Pokémon adventures. For example, he might have to adapt to new clothing styles and different social interactions. His Pokémon might also react in various ways to his new form.