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.
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 common theme could be the exploration of Dawn's character development in relation to her weight gain. It might show how she copes with the physical changes and how her friends and Pokemon react. Another theme could be the challenges she faces in her Pokemon journey due to the weight gain, like being slower in certain situations. Also, it could focus on her self - acceptance journey as she learns to love herself despite the changes.
Her Pokemon are very likely to be involved. For example, Piplup might be very concerned about Dawn's weight gain and try to cheer her up or help her in some way. Her friends like Ash and Brock could also be in the story. Ash might be a bit clueless at first but then show his support, while Brock could offer some advice on healthy eating or dealing with the physical changes.
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 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.
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.
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.
It could be about someone's journey of gaining weight during a new phase or period in their life, perhaps related to a new lifestyle, diet, or health condition.
Pokemon weight gain comics usually have exaggerated depictions of the Pokemon's size and shape. They might also focus on the reactions of other characters to the weight gain. Sometimes, there could be a story element related to how the weight gain occurred.