An engaging plot is key. It could involve new Pokémon battles, unique storylines that tie in with the Sinnoh region. Maybe they discover a hidden power in a Pokémon that changes the course of their journey. Also, the relationship between Ash and Dawn should be developed in an interesting way, like a slow - burn friendship or something more.
One great fanfiction could be 'Ash and Dawn's Unforgettable Sinnoh Adventure'. It really delves into their relationship growth during the Sinnoh journey. The author beautifully describes their battles and how they support each other.
One popular Ash and Dawn Sinnoh fanfiction could be those that focus on their journey through the Sinnoh region, with new adventures not shown in the original series. For example, a fanfiction might have them encounter a rare and powerful Pokemon that changes the course of their journey.
Some potential rivals could be the local Unova trainers who are known for their unique battling styles. For example, a trainer who specializes in using Bug - type Pokemon could be a tough rival for them.
Some Pokemon - focused forums also have sections dedicated to fanfictions. Members of these forums often share and discuss their own works or recommend good ones they've read. It might take a bit more digging, but you could find some hidden gems there.
Ash could develop his character in Sinnoh by becoming more strategic in his battles. Since Sinnoh has a variety of strong and unique Pokemon, he'd need to think more carefully. Misty could develop by becoming more independent. Maybe she takes on some solo adventures in Sinnoh, away from Ash for a bit, and this helps her grow as a trainer.
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.
It could be about a revised or remade version of Ash's adventures in the Sinnoh region in a fan - created story. Maybe it features new storylines, character developments, or different interpretations of the original Sinnoh plot with Ash as the central character.
There's also 'Ash and May in the Sinnoh Wilderness'. This fanfiction focuses on their exploration of the Sinnoh wilderness. They face various challenges together, like dealing with rare and powerful Pokémon. The way the writer describes their bond strengthening as they overcome these difficulties is really engaging. It also has some great descriptions of the Sinnoh landscape.
I'm not sure who the author is. There are so many fanfiction writers out there, and without more information, it's difficult to determine the specific author.