I'm not entirely sure specifically as it could vary greatly. But generally, a 'fanfiction' about Supergirl and Mon - El might explore a relationship that has elements of abuse in the context of the story created by the fan writer. It could be about power imbalances, emotional abuse, or other complex relationship dynamics within the framework of their characters as known from the Supergirl series.
It's hard to say there are positive aspects in terms of the 'abusive' part. However, it could be a way for the writer to show how not to have a relationship. Also, it might be a creative exploration of the darker sides of the characters that we don't usually see in the regular Supergirl story.
Their relationship would likely become more protective. Mon - El would be constantly worried about Kara's well - being and that of the baby. He would be more vigilant and might even become overprotective at times. Kara, on the other hand, might struggle with accepting too much help as she is used to being the one who protects others. But she would also have to learn to rely on Mon - El more, which could strengthen their bond in a new way.
Obviously, Supergirl herself should be included. Her presence can add more drama and conflict, especially if she has mixed feelings about Lena and Mon - El's relationship. J'onn J'onzz could also be a great addition as he can offer his wisdom and alien perspective.
Well, a possible plotline is where Lena starts to develop feelings for Mon - El while he is still trying to figure out his place in the world of Supergirl. There could also be a plot where they get into an argument over the best way to deal with a new alien threat, but later reconcile and come up with an even better plan together.
One possible plot twist could be that Kara's pregnancy somehow affects her superpowers. Maybe she loses them temporarily, which adds a whole new layer of vulnerability and drama. For example, she has to rely on her friends and Mon - El more than ever to protect her and the unborn child. Another twist could be that there are some unexpected genetic mutations in the baby due to their unique alien heritages, leading to a race against time to find a way to ensure the baby's health.
He might leave to protect Kara and the unborn child. Maybe he discovers that there is some kind of danger following him that could put them in jeopardy, so he decides to sacrifice his time with them to keep them safe.
Even though Mon - El leaves, their relationship can still develop. Kara could keep a journal for the baby, writing about her feelings for Mon - El. When Mon - El returns (if he does), he could read it and realize how much Kara truly loved him. Also, Kara might have dreams or visions of Mon - El during her pregnancy, which could show that their connection is still strong on an emotional level.
Love is a very common theme. Since their relationship is a central part of the stories, it often explores different aspects of love like first love, unrequited love, or love that overcomes obstacles.