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.
I'm not entirely sure specifically as 'ash weight gain fanfic' could be very specific to a particular fandom or writer's creative idea. It might involve a character named Ash experiencing some form of weight gain in a fictional story.
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.
His relationships with his Pokemon can also change. If he gains weight and becomes less mobile, some of his more energetic Pokemon might have to take on more protective roles. This could strengthen their bond as they work together in a new way. Also, with his friends, he might become more reliant on them at first, but then grow stronger mentally as he overcomes the challenges related to his weight gain.
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.
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.
One interesting plot could be that Ash discovers a new Pokemon that has the power to make things around it gain weight, and accidentally gets affected himself. This leads to him having to adapt to his new body while still trying to pursue his Pokemon journey.
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.