Another angle could be that her weight gain is part of a magical or medical experiment gone wrong. This could lead to a whole adventure where she has to find a way to reverse it or learn to live with it while also dealing with the political or social implications within the ninja world. For instance, some might see her as a liability, while others might see her as a unique individual with a new perspective.
One possibility is that the fanfic could explore how Sakura's increased weight might affect her ninja training. Maybe she initially struggles with her new body shape, but then discovers new strengths and abilities she didn't have before. For example, she could develop more power in her physical attacks due to increased muscle mass from her weight gain journey.