One storyline could be about Ash finding a lost and injured Pokémon as his new partner. For example, if his new partner is a Togepi. He could nurse it back to health and in the process, form a strong bond. As Togepi grows, it could learn new moves and help Ash in gym battles, especially against fairy - type opponents.
A great storyline would be if Ash's new partner is a Ditto. Ditto can transform into other Pokémon, which could lead to some really interesting plot twists. Maybe Ditto gets captured by Team Rocket thinking it's a rare Pokémon, but Ash has to rescue it. And later, Ditto uses its transformation ability to outwit other villains in battles.
If Ash's new partner is a Hoothoot, the story could revolve around Ash training it to be a great nocturnal fighter. Since Hoothoot is more active at night, Ash could have to adjust his training methods. They could go on adventures in the forest at night, facing other nocturnal Pokémon and solving mysteries. This would also add a new dimension to Ash's journey as he explores the Pokémon world during different times of the day.
Another cool new partner for Ash in fanfiction could be Riolu. Riolu is strong and has the ability to sense auras. It could learn a lot from Ash and Pikachu and grow into a powerful Lucario. This would give Ash a Pokémon with great combat skills and a unique ability that could help in battles and in understanding other Pokémon's emotions.
One possibility could be that in this fanfiction, Ash discovers the Spearow in a unique situation. Maybe it was injured and Ash helped it, leading to a strong bond. The Spearow could then have unique abilities that help Ash in his Pokemon journey, like being able to fly faster than other flying - type Pokemon in his team.
The plot could involve Ash finding the Spearow when it's lost. Ash takes it under his wing and starts training it. The Spearow might be stubborn at first, but as Ash shows patience, it begins to listen. Together, they enter battles and the Spearow surprises everyone with its unique battle style.
Begin with Ash's character. Consider his growth and development throughout the Pokemon series. You could write about a time when Ash has to face his biggest fears in a new Pokemon world. It could be a world where the rules of Pokemon battles are different, forcing him to adapt in new ways.
He could have adventures exploring new regions in the Pokemon Horizons universe that haven't been shown in the main series. Maybe he discovers a hidden island full of rare and unique Pokemon.
A great Ash new Pokemon fanfiction often has a unique plot. For example, Ash could be transported to a parallel Pokemon universe where he has to learn new battle strategies. Good character development is also key. Show how Ash changes and grows in response to new challenges.
It could be a young girl named Lily who has just started her Pokemon journey. She has a Pikachu - like Pokemon but doesn't know how to train it properly. Ash steps in and shows her the ropes, like how to train it to use electric attacks more effectively.
Potential characters might include a new group of Pokémon thieves that Ash has to deal with. Also, there could be a local Pokémon healer who becomes an important part of the story. There might be some young Pokémon enthusiasts who look up to Ash and join him on his adventures. Another possibility is a reclusive Pokémon trainer who has some rare Pokémon and Ash tries to learn from them.
It could be his rivals. For instance, Tracey or Paul might see Ash's potential in the new region as a threat and decide to betray him. They could try to sabotage his battles by giving false tips to his opponents or stealing his battle strategies.
There could be a new rival for Ash. This rival might have a very different approach to Pokemon training, like focusing more on the beauty and grace of Pokemon rather than just battle strength.