There's also a possibility that it's a misanthropic Pokémon that just doesn't like humans interfering in the Pokémon world. It sees Ash as a symbol of human intrusion and decides to kidnap him with the help of the eevee to send a message to all human trainers.
Well, perhaps Pikachu would go on a heroic quest to rescue Ash. It might team up with other Pokémon like Charizard. They could follow the kidnappers' trail, facing various challenges along the way, like battling wild Pokémon or getting through difficult terrains.
In a fanfiction where Ash is reincarnated as an Eevee, his journey would be very different. Firstly, his physical abilities would be limited to what an Eevee can do. He couldn't just run around freely like a human. However, his knowledge of Pokémon battles and the world in general would be a huge advantage. He could potentially form alliances with other Pokémon that he knew were strong from his past life. Also, his relationship with Pikachu would be interesting. Would Pikachu recognize him? And how would Ash, as an Eevee, interact with Pikachu now?
A new villainous trainer group. They might see Ash as a threat to their plans of dominating the Pokémon world, so they kidnap him to stop him from competing in important battles or tournaments.
Ash could use his Pokémon. Pikachu, for example, could use its electric attacks to short - circuit any devices or barriers holding Ash captive. His other Pokémon like Charizard could also help by using their strength or special abilities to break free.
One possible plot could be that Ash is kidnapped by a rival trainer group who want to steal his Pokémon. His friends then have to embark on a rescue mission, facing various challenges and battles along the way.
At first, Ash and Eevee might be a bit cautious around each other. Ash is excited to have caught a new Pokémon, but Eevee is in a new and strange environment. As they start training together, they build trust. Eevee might learn new moves from Ash's guidance, and Ash learns more about Eevee's unique abilities.
His personality might become more cautious. As an Eevee, he's smaller and more vulnerable compared to being a human. So he'd probably be more careful when approaching new situations or other Pokémon. But his determination from his previous life as Ash would still be there. He'd still be eager to prove himself and become strong, just in a different way.
Well, he could be used as a bargaining chip. Team Flare might try to exchange Ash for some powerful Pokémon or valuable items. They could also try to force Ash to reveal any secrets he has about powerful Pokémon or rare items he's come across during his travels.
One great 'Ash starts with Eevee' fanfiction is 'Eevee's Adventure with Ash'. It shows Eevee evolving in unique ways under Ash's guidance. Another one could be 'Ash and Eevee: A New Bond' which focuses on their initial meeting and how they form a strong connection that helps them in battles.