One popular one might be where the author explores the aftermath of Prim's death through Katniss's perspective in great detail. It shows how Katniss copes with the loss and her internal struggle.
First, really understand the characters. Know their personalities, motives, and relationships. For example, for Katniss, her love for Prim is a driving force. Then, plan out the plot. Decide how you want to portray Prim's death and the consequences.
Mockingjay is all about the war and chaos. Katniss is at the center of it, trying to make sense of everything and protect those she loves. The book has lots of twists and turns, and it's a wild ride from start to finish.
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.
In many fanfictions, Prim's location can vary depending on the plot the author creates. Sometimes she might be in a safe haven, while in others she could be in a dangerous situation.
Well, Prim in Hunger Games fanfiction could be anything the author decides. She could have new adventures, face different challenges, or have relationships that weren't shown in the original canon.