Emotional depth is another factor. Since their relationship in the original series is so complex, a good fanfiction will explore their feelings further. Whether it's the love they have for each other, the trauma they've endured, or their hopes for the future. When the author delves deep into these emotions, it creates a more satisfying read.
The plot is crucial. It could be a continuation of their story after the events of the Hunger Games, or a what - if scenario. A unique and engaging plot, like exploring their relationship in a different setting or during a new threat, makes for a great fanfic. Also, good writing quality with proper grammar and description helps bring the story to life.
A great Katniss and Peeta fanfiction often has well - developed characters. The author should stay true to their personalities from the original books. For example, Katniss being strong - willed and Peeta being kind - hearted. If the characters act out of character, it can ruin the story.