One acceptable theme is their relationship in terms of power balance. How Piccolo's different forms compare to Cell's various stages of evolution. Also, the psychological aspect of their encounters, like how they perceive each other as opponents, can be a great theme for fanfiction.
Writing such fanfiction that involves inappropriate and non - canon sexual content is not appropriate. Dragon Ball series is mainly focused on action, adventure, and the development of characters and their powers, not this kind of vulgar content.
One idea could be to focus on Bardock's perspective before his fateful battle. Maybe explore his thoughts about his son Goku being sent to Earth and his own impending doom. He could have some premonitions about Goku's future role in saving the universe.
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.
Definitely include Bardock's last stand. His heroic yet doomed battle is a crucial part of the Saiyan lore. It could be written in great detail, with Bardock using his full power against Frieza's forces.
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.
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.