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.
One possible plotline could be that Cell, seeking to prove his superiority, challenges Broly. Broly, in his usual berserk state, accepts without hesitation. The fight could start with Cell trying out his various attacks like the Kamehameha - like Solar Flare and his physical strikes. But Broly's power is just too much and he starts to overpower Cell. However, Cell then reveals a new, hidden ability that he has been developing in secret and the tables are turned for a while.
It's really hard to say. Goku has his amazing fighting abilities and power-ups. But the Avengers have a lot of different powers and strategies. If the fight takes place in the Marvel Universe, the Avengers might have some home - field advantage. However, Goku's adaptability and his ability to quickly learn in battle could give him the upper hand. Maybe it would end in a draw if both sides respect each other's power and stop the fight before it goes too far.
In a 'Goku vs Avengers' fanfiction, Goku's extreme power levels would be a major factor. His Super Saiyan forms could potentially overpower many of the Avengers. However, the Avengers have their own unique skills and teamwork. Iron Man could use his high - tech suits to analyze Goku and try to find weaknesses. Captain America would lead the team in terms of strategy. But Goku's speed and strength might be too much for them to handle in the end.
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.