There could be a Capitol official who has a strange fascination with the pizza boy. This official might try to manipulate the games to either help or harm the pizza boy, depending on his mood. Also, a tribute from a rich district who initially sees the pizza boy as an easy target but later realizes his unique skills could be an interesting main character. The rich tribute could then form an uneasy alliance with the pizza boy.
A mentor figure who was once a Hunger Games winner could play a significant role. This mentor could see the potential in the pizza boy and try to train him in combat and survival skills. And then there's the little sister of the pizza boy. She could be back home, inspiring the pizza boy to fight and survive, and the thought of getting back to her could be his main motivation throughout the Hunger Games in the fanfic.