Well, one idea could be how Katniss copes with the fear of bringing a child into a post - Hunger Games world. Maybe she'd be worried about the safety of her unborn child in a society that's still recovering. Another could be how Peeta reacts to the news. He might be overjoyed and very protective.
A post - Hunger Games plot could be that Katniss and Prim are trying to help the children of District 12 who have been orphaned or traumatized. They start a small school or a healing center. Katniss teaches the kids survival skills, and Prim uses her medical knowledge to help them heal both physically and emotionally. This could also involve some challenges from the Capitol - loyal remnants in the district who don't want things to change.
One idea could be a what - if scenario where Katniss and Clove form an unlikely alliance in the arena. Maybe they realize that the Capitol is the real enemy and decide to team up against the other tributes. Another plot could involve Clove having a change of heart towards Katniss after seeing her kindness towards Rue. And a third idea is that they both survive the Hunger Games and have to deal with the aftermath together, facing the Capitol's wrath and trying to start a rebellion from within.
Another plot could be set during the Quarter Quell. Peeta and Katniss find out that there are some Capitol secrets hidden in the arena. They then team up not only to survive but also to expose these secrets. Their relationship would be put to the test as they face new and more dangerous threats.
A fanfic could explore a scenario where Finnick saves Katniss's life in a different way. Instead of in the arena, it could be in a Capitol raid. Finnick uses his knowledge of the Capitol's layout and his combat skills to keep Katniss safe. This could lead to a deeper connection between them as Katniss is indebted to Finnick for her life.
In a darker fanfiction, Katniss could be assassinated. The Capitol, still angry about her rebellious acts, sends a highly skilled assassin. The assassin manages to infiltrate District 12 and kill her in her sleep. This would lead to a huge uprising as Katniss was seen as a symbol of hope for the districts.
Some fanfictions focus on the medical aspect of Katniss being pregnant. Since they are in a post - apocalyptic world, medical resources are scarce. So, it could be about how they manage to get proper medical care for Katniss during her pregnancy. Another theme is the cultural impact. In the Hunger Games world, there are different districts with their own cultures. How does Katniss's pregnancy affect or is affected by these cultures? And finally, there's the theme of protection. Who will protect Katniss and her unborn child from potential threats, both internal and external?
Well, for Hunger Games pregnant fanfic, you could write about a female tribute who was already pregnant before the Games but kept it a secret. As the Games progress, she starts to show signs of pregnancy, which both confuses and intrigues the other tributes. Or perhaps a story about a Capitol citizen getting pregnant by a tribute she was supposed to be mentoring, and how that causes a scandal in the Capitol society. Also, a fanfic where a pregnant character teams up with other tributes not for the sake of winning but for the sake of protecting her unborn child could be really interesting.
Maybe the OC is pregnant and it's a result of a forced situation in the Capitol. This could add a whole new layer of motivation for her in the Hunger Games. She could be fighting not just for her own survival, but also for the future of her unborn child. And she might form alliances with other tributes who are sympathetic to her situation.
One interesting fanfiction could be about Peeta and Katniss' life after the games in a peaceful district. They could be rebuilding their community and dealing with the emotional scars of the Hunger Games together.
One interesting scenario could be Katniss and Gale's adventures before the Hunger Games start. Maybe they are out hunting together in the woods and encounter a rare and dangerous animal that they have to figure out how to deal with. It would show their survival skills and how they rely on each other.