The main character might be a boy named Billy. Billy is a bit of a glutton and he doesn't know when to stop eating. So, in the fanfic, his overeating habit could be the cause of his belly bloating. For example, he enters an eating competition and eats way too much, resulting in a very bloated belly.
Well, it could be about a boy who eats a lot of strange foods at a magical feast and his belly starts bloating in a very exaggerated way. Maybe there are some fairies or magical creatures around him who are both amused and concerned about his bloated state.
Since I don't know the details of the 'belly gainer boy story', I can't say for sure who the main character is. It's probably a boy, as the title suggests.
Well, 'bloating fanfic' can be thought of as fanfiction that has gotten out of hand in terms of its length or content. Maybe the writer is trying too hard to expand on the original source material's world or characters. It could end up making the story feel cumbersome and less engaging. For example, if a fanfic about a certain movie character starts adding dozens of side characters and sub - plots that don't really add much to the main story, it becomes bloated.
An inventor could be the main character. While working on an experiment to create a new type of super - food that provides a huge amount of energy, he accidentally consumes some of it. This causes his belly to start expanding uncontrollably. He then has to use his inventive skills to find a way to reverse the effect while also dealing with the practical problems of having a large belly, like getting around his cluttered laboratory.
It could be a jealous rival of Bucky. Maybe this rival is envious of Bucky's popularity and when he sees Bucky's round belly, he uses it as an opportunity to mock and undermine him at every turn, becoming the main source of conflict in the fanfic.
Sharon Carter could be in the fanfic. She could be the one who provides some medical or scientific insights into what's happening to Bucky. Maybe she has some knowledge of the experimental substances that might be related to Bucky's growing belly and tries to use her resources to help him.
One reason could be diet. If a boy consumes a lot of high - calorie foods like junk food and sugary drinks regularly without much exercise, he is likely to gain belly fat. For example, eating a lot of fried chicken and soda.
The obvious main character would be the delivery boy himself. He could be a young guy just trying to make some money to support his family or pay for his studies.
The Penguin could be a great main villain. He could be up to his usual schemes in Gotham, like stealing valuable artifacts. When the little boy witnesses one of his crimes, the Penguin tries to silence him. Batgirl then has to protect the boy and stop the Penguin's evil plans.
Slade could be a great main villain. His combat skills and strategic mind would pose a huge challenge for Beast Boy. Beast Boy would have to constantly outwit and outmaneuver Slade, which would really bring out his badass side as he tries to protect his friends and the city from Slade's evil plans.