Well, a cool plot might be that Katniss and Peeta find themselves in a part of the arena that was never used before in the official Hunger Games. This new area has strange creatures and challenges that they must face together. They could also be forced to make a difficult choice between helping other tributes or focusing on their own survival, which would really test their characters. And, a plot where they have to deal with a disease outbreak in District 12 and use their skills from the Games to help the community would be interesting too.
A possible plot could be that Katniss and Peeta are kidnapped by a group who wants to use them for their own political gain. They have to escape and find their way back to their families and the districts they love. Along the way, they might face internal conflicts as they deal with their feelings for each other. Another idea is that they start a rebellion within the Capitol itself, using their fame and the knowledge they gained from the Games to incite change. Also, a plot where they discover a hidden power or ability within themselves during a time of crisis could be really engaging.
One plot idea could be Katniss and Peeta having to survive in a post - Hunger Games world filled with new threats that force them to rely on each other even more. Another could be a story where they go on a secret mission together outside of District 12 and discover something that changes their relationship. Or perhaps a plot where they have to deal with a false rumor about their relationship that causes chaos in the districts.
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.
One way is to have them face more personal challenges together. For example, if one of them gets injured badly, the other has to take care of them and in the process, their bond deepens. Another way is to introduce a third party who is interested in one of them romantically, which makes them realize their true feelings for each other more clearly.
One idea could be that Katniss, Peeta and Lemon find themselves in a situation where they have to work together to survive in a new, uncharted territory. Maybe there's a new type of danger that the Capitol has created and they have to use their combined skills to overcome it.
Maybe the baby has some special abilities or characteristics that are unique to the world they live in. For example, the baby could have an uncanny ability to sense danger, like Katniss with her hunting skills. This would add an interesting dynamic as Katniss and Peeta have to figure out how to handle this while also being new parents.
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 plotline could be an alternate version of their time in the arena. Maybe they find a hidden part of the arena that has some kind of special power or resource that changes the game completely. For example, a place with healing waters that not only heals their physical wounds but also gives them some mental clarity, allowing them to outwit the other tributes more easily.
One interesting plotline could be Peeta and Katniss having to survive in a different kind of arena that has unique challenges not seen in the original games. Maybe it's an underwater arena where they have to figure out how to breathe and find food. Another could be a plot where they secretly raise a rebellion within the Capitol itself instead of just in the districts.
One interesting fanfiction could be about Katniss and Peeta's life after the games in a peaceful district. They start a school for the children to teach them survival skills in a non - violent way. It shows their growth from being survivors of the brutal games to educators.
Another plot could be that Peeta's memories, which were once messed with, start to come back in a really strange way. He has flashbacks that are a mix of real and false memories, and Katniss has to help him sort through it all. This creates a lot of tension and emotional turmoil in their relationship as they try to figure out what's real and what's not.
In some fanfiction, a great plotline is that Peeta and Katniss are forced to participate in a special Hunger Games - like event again, but this time it's more of a political ploy. They have to outwit the Capitol in different ways than before. Maybe they form alliances with unexpected people from other districts and find new ways to resist the Capitol's tyranny while also dealing with their complex feelings for each other.