One possible plotline could be Ginny struggling with body image at first. Maybe she starts to gain weight due to a spell gone wrong. But then she realizes that her true friends don't care about her appearance and she learns to love herself as she is.
Another plotline might involve Ginny's weight gain being part of a magical transformation that she has to endure for a greater good. For example, she gains weight as she absorbs some powerful magic, and during this process, she discovers new abilities within herself. Along the way, her relationship with Harry could be tested as he has to come to terms with the change too.
One interesting story could be about Harry Potter himself suddenly gaining weight due to a magical mishap in the Hogwarts kitchens. Maybe a spell on the food backfires and he starts putting on pounds while trying to figure out how to reverse it.
It could strengthen her relationship with Harry. Harry, being a true friend and lover, would support Ginny through her change and show that his love is not based on appearance. This would deepen their bond.
A common plotline is Ginny's role in the post - Voldemort wizarding world. For example, in some fanfictions, she helps to rebuild Hogwarts or plays a key part in the new Ministry of Magic. Another plotline could be Ginny's relationship with her siblings in a more in - depth way, like dealing with family secrets or rivalries.
In some fanfictions, there are plotlines about the Potter family traveling to different magical places around the world. They might encounter new magical creatures or ancient magic. For example, they could go to a hidden magical island where the magic is so pure and powerful that it's both a wonder and a danger. And it could be a story about how they adapt to these new situations while also teaching their kids about the wider wizarding world.
Well, perhaps in the fanfiction, Ginny might be forced by some external power to make Harry do things he doesn't want to. For example, she could be made to persuade Harry to take part in a dangerous mission that he initially refuses. Or it could be about Ginny's overzealous attempts to get closer to Harry, which Harry feels are forced, creating a rift between them that they need to overcome.
One interesting 'Harry Potter male weight gain fanfiction' could be where Neville Longbottom, after spending a lot of time in the Hogwarts kitchens, starts to gain some weight in a rather endearing way. Maybe he discovers new magical plants that have a side effect of increasing appetite.
A great plotline might involve Ginny using her Quidditch skills to start a new team or training program at Hogwarts, and Harry supporting her in this endeavor. They could also face some sort of mystery related to an old Hogwarts secret that only they, with their combined knowledge of the school's history, can solve.
In some fanfictions, there is a plotline about how their friends and family react. Ron might be his usual bumbling self but also very excited. Hermione could be the super organized friend who helps Ginny with all the preparations. And then there could be a plotline where they have to protect Ginny and the unborn baby from some dark forces that are still lurking in the wizarding world, fearing the power the Potter child might possess in the future.
Well, in some fanfictions, a great plotline is them raising their children. Harry, with his past traumas, and Ginny, with her strong Weasley family values, might have different ideas on how to bring up their kids. For example, Harry might be overly protective at times due to his own experiences, while Ginny wants to encourage their children's independence and exploration of magic.
A plotline might involve her relationships with other characters. For example, Jaime might have a different reaction to a heavier Cersei. He could be more protective or, on the contrary, feel a sense of distance due to the change. And her enemies might underestimate her because of her weight gain, only to be outwitted by her.