I think Grookey is a great starter for the Pokemon Sword story. It has a strong start in terms of its attacks and evolves into a very powerful final form. It can easily handle the early - game battles which are important for progressing through the story.
In this fanfiction, Ash and Dawn's adventures in Unova could be filled with new Pokemon encounters. Ash might bring his typical enthusiasm and determination, while Dawn could add her own unique charm and Contest skills. They could team up to face new challenges, like strong Gym Leaders in Unova. Their friendship would also likely develop further as they explore this new region together.
In a fanfiction of Ash in the Unova League, it could focus on his relationships with his Pokemon during this journey. Maybe Pikachu has a special moment of growth, or he catches a new and rare Pokemon that becomes crucial in his battles in the league.
In a Pokemon fanfiction with Ash and Mewtwo, it could start with Ash encountering Mewtwo in an unexpected place. Maybe in a hidden forest where Mewtwo is observing the wild Pokemon. Ash, being his curious self, tries to approach Mewtwo. Mewtwo, initially hostile, realizes Ash's pure intentions and they start an unlikely friendship. This could lead to many adventures where Ash and Mewtwo face various challenges together, like battling against a new, powerful and evil Pokemon organization.
To write a compelling Eevee starter character, first give it a distinct personality. For example, it could be a curious Eevee that is always getting into trouble because it wants to explore. Second, give it a unique backstory. Maybe it was abandoned by its previous trainer and is now a bit shy but determined to prove itself. Third, show its growth throughout the story. As it faces challenges, it becomes more confident and learns new skills.
The main character from the Pokemon Black and White games, Hilbert or Hilda, are also quite popular. Fanfic writers like to expand on their adventures in Unova, give them new challenges and relationships. Also, characters like Team Plasma members are popular as they can be portrayed in different ways, either as villains with more depth or even as characters who have a change of heart in the fanfic.
One interesting storyline could be about a trainer who discovers a rare and powerful Pokemon in the Unova region that has been lost for centuries. This Pokemon could have unique abilities that change the way battles are fought in the fanfic.
In the Unova League in the Pokemon series, Ash winning is quite an exciting concept in fanfiction. It shows his growth as a trainer. Maybe in this fanfiction, he finally overcame all his opponents with his strong team, strategic thinking and unwavering determination.