One possible storyline could be that Harry leaves Ginny due to some external threat that he doesn't want to endanger her and the unborn child. For example, he might have discovered a new dark wizard on the rise and decides to go into hiding alone to draw the danger away from his family.
Maybe Harry leaves because he has some self - doubt. He could feel that he's not ready to be a father and is afraid that his past traumas and the fame that comes with being 'The Boy Who Lived' will have a negative impact on the child. So, in a moment of panic, he runs away, leaving Ginny pregnant and heartbroken.
One possible plotline could be Ginny leaving Harry due to the pressure of his fame. She might feel overshadowed and decides to pursue her own life away from the spotlight. Another could be a misunderstanding. Maybe she misinterprets Harry's actions or words and in a fit of anger, leaves him. Or perhaps an external threat forces her to leave to protect Harry, like a dark wizard threatening her to stay away from him so as not to be used against Harry.
Well, in this kind of fanfiction, a plot development could be that Ginny, being pregnant, discovers new magical abilities or powers related to motherhood. This could be a unique take on the story. Also, the wizarding community might react in different ways to Ginny's situation. Some could be supportive, while others might shun her because of Harry's departure. It could lead to Ginny having to fight for her place in the community and prove that she can be a great mother on her own terms.
Perhaps in the fanfic, Ron is away on some important mission when Ginny discovers she's pregnant. When he returns, he has to quickly adapt to the situation. They then decide to keep the pregnancy a secret for a while to avoid unwanted attention and drama within the wizarding community.
One possible storyline could be that Harry is under the influence of a dark spell that makes him act out of character and be unfaithful. Another could be that he gets caught up in a dangerous mission where he has to pretend to be involved with someone else for the sake of secrecy and Ginny misinterprets it as cheating. Maybe there's a new character who manipulates Harry into thinking he has feelings for her.
One possible reason could be that Harry realizes he has stronger emotional and intellectual compatibility with Ginny. He might find that Ginny understands his complex life as a wizard better, especially with her own experiences in the wizarding world. Also, Ginny is more in tune with Harry's adventurous spirit, which Cho might not be as much.
She might also be sad and heartbroken. Ginny is a strong character, but being left alone while pregnant is a huge blow. She could withdraw from her friends and family for a while as she tries to come to terms with the situation and figure out how to raise the child on her own.
Well, it could be that Ginny is pregnant in a world where magic exists (assuming this is related to a magical universe like Harry Potter if Ginny is from there). The pregnancy could bring about some sort of magical phenomenon. Maybe the unborn baby has some sort of special power that starts to manifest during the pregnancy. This could lead to Ginny having to protect the baby from those who might want to use the baby's power for their own gain. At the same time, Ginny has to deal with the normal physical and emotional aspects of pregnancy like morning sickness and mood swings.
Perhaps Ginny discovers some hidden secrets or powers within herself as she copes with Harry's death and the pregnancy. Maybe she has to deal with people who question her ability to raise the child without Harry. The story could also focus on how the child grows up with the legacy of Harry, and whether the child inherits any of Harry's special abilities or traits.
Another storyline could involve the reactions of their friends. Hermione and Ron might be there to support Ginny throughout her pregnancy. There could be a lot of scenes of them sharing their experiences and knowledge about having a baby, adding a touch of friendship and camaraderie to the story.
In this fanfiction, Ginny might find strength within herself that she didn't know she had. She could start to form new relationships with other characters who help her through the pregnancy. For example, she could become closer to Hermione, who could offer practical and emotional support. The pregnancy could also be a catalyst for changes in the wizarding society's view on single motherhood. It could lead to new laws or social norms being established, and Ginny could be at the forefront of this change.