A common theme is the exploration of the Pokémon world from a different perspective. As an Eevee, Ash sees things in a new light. He experiences the Pokémon world on a more intimate level, like understanding the language of Pokémon better. There's also the theme of adaptation. He has to quickly adapt to his new physical form, which includes learning to move, run, and use his new Eevee - specific abilities.
Another theme is Eevee's evolution. Many fanfictions explore different ways Eevee could evolve and how that affects its relationship with Ash and their adventures. For example, if Eevee evolves into a Vaporeon, it might lead to underwater adventures. Also, the theme of friendship and loyalty is always there. Ash and Eevee rely on each other, and this is often a central part of the stories.
A common theme is growth. Both Ash and the Eevee grow as individuals. Ash becomes a better trainer, and Eevee evolves or becomes stronger. Another theme is friendship. The bond between Ash and Eevee is often explored deeply.
Another theme is growth. Ash might help Eevee evolve or Eevee might help Ash become a better trainer. For example, in some fanfictions, Eevee's loyalty gives Ash the motivation to keep going in tough battles. Also, the theme of friendship that turns into love is quite prevalent. They start as friends but as they share more experiences, their relationship deepens into love. It's really interesting to see how different authors explore these themes in different ways.
The plot might involve Ash turning into an Eevee and being found by a group of Eevee and their evolutions. They initially view him as an oddity since he has the memories and some of the behaviors of a human. Ash, as an Eevee, could then learn new skills from his new companions and use his knowledge from being a human trainer to help the group in various situations, such as finding food sources or avoiding predators.
Well, in these fanfictions, a frequent theme is identity. Ash has to grapple with his new identity as Arceus. He's no longer just a regular Pokémon trainer. Also, the theme of sacrifice is common. There's usually something that led to his transformation, which might have involved sacrificing something he held dear. Additionally, many of these fanfictions touch on the theme of change. His transformation into Arceus brings about changes in his relationships with his Pokémon friends and other trainers in the world.
I'm not sure of the exact name, but there's a fanfiction where Ash turns into an Eevee and gets caught by a trainer. This one is really unique as it shows the Pokémon world from the perspective of a captured Pokémon. Ash has to use his knowledge as a human - turned - Eevee to escape and also tries to communicate with the other Pokémon in the trainer's team. It's a great read for those who like a different take on the 'Ash turns into an Eevee' concept.
His other Pokémon like Charizard would be completely bewildered. Charizard might initially think that this Eevee is just some random Pokémon near its territory. But when it senses something familiar about the Eevee, it would be hesitant and curious. It could try to communicate in its own way, like using small bursts of fire to get the Eevee's (Ash's) attention. The Pokémon that Ash has a closer bond with would be more determined to figure out what's going on and how to help him.
Well, often there's a theme of change. The pregnant Eevee has to adapt to physical and emotional changes. There can also be a theme of community. The other Pokémon around the pregnant Eevee might form a sort of community to support it. For instance, they could help gather food or keep the area safe. In some fanfictions, the theme of mystery might also be present. There could be something special or unique about this particular Eevee's pregnancy that the characters need to figure out.
Friendship is also a big theme. The female Eevee might make friends with other Pokémon or even with human trainers. These friendships can be the central focus, showing how they support each other through various situations.
A common theme is the exploration of new powers. Since Ash becomes a Lucario, the fanfics often focus on him learning to use abilities like Aura Sphere or Extreme Speed. Another theme is identity. Ash has to come to terms with being a different Pokemon and how that changes his relationship with his friends and his role in the Pokemon world.