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.
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.
Well, Goku's Saiyan powers are all about brute strength, speed, and the ability to transform into more powerful forms. Android 21 has her unique android abilities like energy absorption and regeneration. If they combined their powers, they could create some really powerful attacks. For example, Goku could charge up a Kamehameha while Android 21 absorbs energy from the surrounding environment to make the attack even stronger. Their combined speed would also make them a formidable duo in battle.
In a Naruto - Goku fusion, there could be a new form of energy manipulation. Goku manipulates ki, and Naruto uses chakra. They might create a hybrid energy that has the best of both worlds. This energy could be used for long - range attacks like Goku's energy blasts and also for close - combat like Naruto's Rasengan. Also, the fusion could have enhanced senses, borrowing from Goku's Saiyan senses and Naruto's ninja training.
Another plot could involve Female Cell having a premonition of a great threat to the universe. She approaches Goku because she believes his power and pure heart can help. Together, they embark on a journey to find the source of this threat. Along the way, they face various challenges that force them to rely on each other, leading to a growing bond between them.