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.
One possible storyline could be that Goku is accidentally transported to Alucard's world. They start off as enemies but then gradually learn about each other's powers and come to respect one another. Another could be that a powerful villain manipulates them into fighting each other. In this case, they might realize they are being used and team up against the real enemy.
It's really hard to say. Goku is extremely powerful with his Super Saiyan forms and incredible combat skills. But Alucard has his vampiric powers like immortality and the ability to control shadows. In a fanfic, it could go either way depending on how the author wants to write it. Maybe if the author wants to emphasize speed, Goku might have an edge. But if it's about dark, mysterious powers, Alucard could come out on top.
One possible scenario could be that Goku discovers a new power within himself during the Cell Games that allows him to not only survive but also defeat Cell more easily. Maybe it's a hidden form that he unlocks at the last moment.
There are some 'cell x goku fanfic' that focus on the power dynamics between them. For example, a fic could show how Goku's Saiyan nature and his love for a good fight might interact with Cell's own desire for combat and self - improvement. This can lead to some really interesting battles and character development.
One idea could be that Cell somehow gets a second chance at life and instead of being a villain, he tries to team up with Goku. They could go on adventures together across different universes, facing new threats and learning from each other. Goku could teach Cell about the value of protecting the innocent, and Cell could show Goku some of his scientific knowledge about power levels and energy manipulation.
It could be Goku. He has always been the main hero in the Dragon Ball series. His unwavering sense of justice and his eagerness to protect the Earth would likely make him take the lead in most situations. Also, his combat experience and his ability to quickly adapt to new opponents would give him an edge in any interaction with Cell.
If Goku was brought back after the Cell Games, it could have led to unexpected alliances or rivalries. Also, his presence might have prevented some future villains from emerging or made their plans much more difficult.