One possible fanfic could be about Goku's intense training before the Cell Games. He might be shown pushing his limits in the Hyperbolic Time Chamber, trying out new techniques and forms to defeat Cell. Maybe he discovers a new power level that no one, including himself, expected.
It could be someone from Goku's past who has a vendetta against him and takes advantage of the chaos during the Cell Games. For instance, an old enemy who was thought to be defeated but managed to come back stronger and wants to finish Goku off while he's weakened from the fight with Cell.
One common theme is the clash of ideologies. Goku is all about protecting the innocent and having a fair fight, while Cell is more about self - preservation and domination at first.
Perhaps a rogue Saiyan from another part of the universe. This Saiyan has heard about the Super Saiyan powers on Earth and wants to prove his superiority. He shows up at the Cell Games and challenges Goku and Gohan. He has a different fighting style, more brutal and less honorable than the Saiyans on Earth. His presence adds a new layer of conflict as Goku and Gohan have to decide whether to fight him on his terms or uphold their own values.
It could be that a new threat emerges. Maybe another android, stronger than Cell, was observing the fight and decides to take advantage of the chaos after Gohan's death. This new android could become the main antagonist as it poses a greater danger to the remaining fighters.
One possible plot could be a story where Goku meets a female version of Cell who has different motives than the original Cell. Maybe she doesn't want to destroy the Earth but instead is on a journey of self - discovery. Goku, being the protector, is initially cautious but then starts to see her potential for good.
Perhaps Gohan sacrifices himself to save the other Z - fighters. He might choose to take on Cell's full - power attack that was meant for all of them, and in the process, he loses his life. In the fanfiction, it could also be that Cell has some sort of hidden power - up that catches Gohan off - guard and leads to his death.
In this fanfiction, if Gohan dies in the Cell Games, perhaps the story could focus on how the other characters cope with the loss. For example, Chi - Chi could become a more hardened character, vowing revenge on Cell or any other threats. Piccolo might take on a more fatherly role to Goten, trying to train him to be as strong as Gohan was. And the Earth might be in a state of mourning, which could attract new villains who see it as a time of weakness.