One possible plotline could be Ash starting his journey in Sinnoh and quickly befriending a rare Pokémon that is shunned by the local trainers. He then has to prove the Pokémon's worth. For example, it could be a shiny Zubat that is thought to bring bad luck. Ash, with his determination, shows everyone that it can be a powerful and loyal partner.
His first Sinnoh Pokémon could be a Chimchar. Imagine Ash finding Chimchar being bullied by some other Pokémon. Ash steps in and Chimchar decides to join him. Chimchar's energetic and mischievous nature would add a lot of fun to Ash's Sinnoh adventures as it grows and evolves into an Infernape.
One interesting aspect could be the character development of Ash in the Sinnoh League within fanfiction. Writers might explore his relationships with his Pokémon in greater depth, like how his Pikachu might face unique challenges. Another could be the battles. Fanfictions might create new and exciting battle scenarios that we didn't see in the original Sinnoh League arc. Also, they could introduce new characters or give more backstory to the existing ones in the Sinnoh League setting.
In a fanfiction where Ash starts in a different region, for example, Sinnoh. He could start with a Pokémon native to Sinnoh like Piplup. His adventure would be shaped by the mythology and legends of Sinnoh. He might get involved in the search for the Legendary Pokémon related to the creation of the Sinnoh region. This would not only add a new layer of mystery to his journey but also expose him to different kinds of Pokémon trainers and their beliefs. His relationships with his Pokémon would also be influenced by the Sinnoh - specific Pokéballs and items available in the region.
Well, one interesting plot point could be Ash encountering new and rare Pokémon in Sinnoh that he's never seen before. Maybe he discovers a unique form of a Pokémon that's native to Sinnoh. Another could be his interactions with the Sinnoh gym leaders, like having intense battles or forming unexpected friendships with them.
In some fanfictions, Ash and May might get involved in a mystery in Sinnoh. For example, strange events are occurring that are affecting Pokémon behavior. They work together to solve the mystery, with Ash using his battle skills and May her knowledge of Pokémon contests. As they dig deeper, they find that an ancient power from Sinnoh's past is being misused.
Perhaps there is a threat to Sinnoh that only Ash's old Pokémon can help solve. Some kind of ancient evil that has knowledge of Ash's past Pokémon battles. Ash and his old Pokémon then have to work together to defeat this threat. They might need to combine their old strategies with new ones they learn in Sinnoh to succeed. And along the way, Ash might meet some of his old friends who are also in Sinnoh and they help him in his mission.
A plotline might be that Latios is from a different dimension or time, and it has come to Ash's world for a specific purpose. Ash gets involved in this mission, and together they have to face powerful enemies. This could introduce new Pokémon species from other dimensions and new battle strategies that Ash has to learn to keep up with the challenges.
One plot idea could be Ash and Misty getting separated from their group in Sinnoh and having to rely on each other to survive in a strange, new area. They could encounter a rare and powerful Pokemon that neither of them has seen before. Another idea is that they discover an ancient mystery in Sinnoh related to a type of Pokemon evolution that was thought to be lost. This could lead them on a journey to uncover the truth and help a particular Pokemon evolve in a new way.
One possible plot could be that Ash discovers a lost Axew in a mysterious forest. The Axew is scared and alone, and Ash decides to take it under his wing. Together, they embark on a journey to find Axew's family. Along the way, they face various challenges like battling other Pokémon and traversing difficult terrains. Ash uses his experience and Pikachu's help to protect Axew.
Perhaps in this fanfiction, Ash has formed stronger bonds with his Pokémon. These bonds allow them to perform at their best in battles. His Pikachu might have learned a new and super - effective move. Also, Ash could have received some secret training from a Sinnoh Pokémon master who taught him unique battle techniques. This would all contribute to his victory in the Sinnoh League.