One common theme could be self - discovery. As Serena gains weight, she might learn more about herself and how she feels about her changing body. Another theme could be the reactions of her friends and Pokemon, like how they support or react to her new look.
I'm not sure of specific 'Pokemon Serena weight gain fanfic' stories. You could try searching on fanfic websites like FanFiction.net or Archive of Our Own. There might be a variety of stories there.
Another theme is the change in their abilities. Sometimes, the weight gain can either enhance or hinder their pokemon powers, which is often explored in these fanfics. For example, a heavier pokemon might be slower but have more physical strength in a fight. Also, friendship is a big theme. Their friends' reactions to the weight gain, whether it's supportive or not, play a major role in the story. In some fanfics, friends help the pokemon to lose weight if it's causing problems, or they accept the pokemon just as it is.
One common theme might be the transformation aspect. Since it's about weight gain, it could explore how Dawn's character changes as a result, both physically and perhaps in her self - perception.
Obviously, Ash is the central character. Pikachu is also often included as Ash's closest companion. Misty and Brock might also be present. They could be the ones who first notice Ash's weight gain and either tease him or try to help him. Other trainers like Gary could also be in the story, perhaps competing with a now - heavier Ash and making snide remarks about his changed physique.
I'm not sure of specific 'pokemon dawn weight gain fanfic' to recommend directly. You could try searching on fanfiction websites like FanFiction.net or Archive of Our Own. They have a large collection of Pokemon - related fanfics, and you might be able to find what you're looking for there.
One interesting plot could be Ash trying different Pokemon foods that cause unexpected weight gain. For example, he might taste a new type of Poke - Puff that has a strange side effect on his metabolism. Another plot could be that a certain Pokemon's ability accidentally affects Ash, making him gain weight gradually. Maybe a psychic - type Pokemon's power goes awry and messes with Ash's body composition.
One popular 'pokemon weight gain fanfic' could be 'The Chubby Pikachu Adventure'. It's a cute story where Pikachu starts to gain weight in a rather humorous way. The author describes Pikachu's daily struggles and how its friends react to the change.
One common theme is the change in the Pokemon's lifestyle. For example, if a Pokemon starts living in a place with abundant food, it may gain weight. Another theme is how the trainer reacts to the Pokemon's weight gain, whether they are concerned or accepting.
Often, there can be themes of self - acceptance. The character who is experiencing weight gain might have to come to terms with their new body shape, and it could be a journey of learning to love themselves. For example, Naruto might initially struggle with his weight gain but then find strength in his new form.