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.
It could be the student council leaders. They are in a position of power already, much like the Careers in the Hunger Games. But instead of using their power for selfish reasons, they could be the ones trying to stop a Hunger Games - style event from happening at the school. They would have to face off against other students who want to start this dangerous game, and they would need to use their leadership skills and influence to protect the school.
The main character could be the shy and nerdy student who no one expects to do well but has hidden strengths. Maybe a girl who is really good at math and uses her logical thinking in the challenges. Another could be the popular jock who realizes that popularity doesn't mean much in this new and dangerous situation. He has to rely on his physical skills as well as learn to work with others he normally wouldn't associate with.
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.
A girl from District 11 who is an expert at climbing trees and has a strong sense of justice. She could be one of the main characters, using her skills to survive in the arena and also standing up for the weaker tributes.
Graystripe could also be a main character. He's a strong and loyal cat. His friendship with Firestar could play an important role in the story. Maybe they are both in the 'games' and have to rely on each other. He could also make new friends among the other cats in the arena. And his love for Silverstream could give him an extra motivation to survive and get back to his clan. There could be flashbacks about their relationship during the fanfiction.