Katniss would likely still be a central figure. She was the face of the revolution, and after the war, she could be leading the efforts to rebuild or at least be a symbol of hope for the people in the districts.
One possible plotline could be about the rebuilding of the districts. The characters might focus on creating a more equal and just society, dealing with the aftermath of the destruction. For example, they could be establishing new laws and educational systems.
Haymitch could be an interesting focus. He has a lot of history and secrets. You could explore his role in the post - Hunger Games world, like how he mentors the next generation or deals with his own demons. Finnick is another one. His charm and the pain he has endured in the games and his life could be delved into deeper, for example, how he uses his popularity to make a difference in the new Panem.
The main characters could be the school's misfits. You know, the ones who don't really fit into the popular groups. They could be like the underdogs in the Hunger Games, and as the story progresses, they show their true strength and courage. Maybe there's a shy bookworm who turns out to be a strategic mastermind, or a scrawny athlete who has unexpected fighting skills.
A nerdy student who has been underestimated all his life could be a main character. He uses his intelligence to outwit the more physically strong opponents. For example, he could create traps using his engineering knowledge.
There could be a Capitol - trained healer who is sent into the Hunger Games as a form of punishment. He has knowledge about the blight and its effects, which could be both an advantage and a disadvantage in the arena. His main goal becomes survival rather than following the Capitol's cruel rules.
Perhaps an older tribute from a previously overlooked district. He has been observing the Hunger Games for years and has a secret plan. He has a lot of knowledge about the Capitol's tactics and uses this to outwit the Gamemakers and survive in the 100th Hunger Games.
A boy from District 9 who is a genius at strategizing. He might have studied the previous Hunger Games extensively and uses this knowledge to form alliances and plan his every move. He could be a complex character, torn between his own survival and his moral compass.
A new character could be the main one. Maybe a girl from a fishing district who has excellent swimming and diving skills. She could be a tough competitor, knowing how to navigate the beach and the sea better than others. Or it could be a boy who was exiled to the beach before the Games started and has learned all the secrets of the beach, like where the fresh water sources are and the safest places to hide from other tributes.
A street - smart orphan from the slums of Coruscant could be a main character. They would have grown up tough and know how to fend for themselves. Another could be a Capitol - bred citizen who volunteers for the Games out of a sense of adventure or to prove something. Maybe they think they can handle it because of their privileged upbringing but soon realize it's not as easy as they thought.
One main character could be a young girl from District 9, who is really intelligent and uses her wits to survive. She might form an alliance with a boy from District 12, who is known for his speed and agility. They make an unlikely duo but their combined skills help them in the games. There could also be an older tribute from District 7, who has a lot of experience in the forests and uses that to his advantage in the arena.