There could be a storyline where Issei challenges Goku to a friendly sparring match to test his own limits. As they fight, they exchange techniques and learn from each other. This could lead to Issei unlocking new forms of his own power, inspired by Goku's Super Saiyan forms. And Goku could also gain new insights into combat from Issei's more unorthodox fighting style.
It depends on the rules set in the fan - fiction. If it's a battle based on strategy, Issei might have a chance. He has shown to be quite cunning in his battles. But if it's a pure power - based fight, Goku has the upper hand. His years of training under various masters and his Saiyan heritage give him an edge. Also, Goku has experience fighting opponents much stronger than himself and finding ways to overcome them.
One interesting storyline could be that Goku, being the ever - eager fighter, challenges Saitama not knowing the true extent of his power. Saitama, on the other hand, is just looking for a normal grocery shopping day. The clash of their personalities, Goku's excitement and Saitama's nonchalance, could lead to a very comical start. Then as the fight progresses, they both start to respect each other's power in different ways.
One interesting storyline could be a plant and a zombie forming an unlikely friendship. Maybe a sunflower and a regular zombie find that they both have a love for music. The sunflower shares her sunlight with the zombie so he can dance to his favorite tunes. It's a really unique take on the whole plants vs zombies concept.
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.
One possible storyline could be that Naruto is somehow transported to Goku Black's universe. There, he has to face Goku Black who sees him as an intruder. Naruto would have to quickly learn about Goku Black's powers and find a way to fight back using his ninja skills. Another storyline could be that a powerful villain creates a rift between their two universes and pits them against each other. They start off as enemies but then realize they are being manipulated and team up to fight the real villain.
There could be a storyline where Beerus, out of boredom, decides to train Goku Black instead of fighting him. But Goku Black misinterprets this as a sign of weakness and attacks Beerus. This could lead to a battle where Beerus has to hold back at first to avoid killing Goku Black, but as the fight progresses, he realizes he has to use his full power to subdue him.
A really cool storyline might be a situation where they are both framed for a crime they didn't commit. They have to work together to clear their names while also dealing with their own personal issues. Batman might be suspicious of Green Arrow at first, and vice versa. This would create a lot of tension and drama. Also, a fanfiction where they are transported to an alternate universe and have to find their way back could be exciting. They would face new challenges and enemies that test their abilities in different ways.
One possible storyline could be that Goku Black arrives on Earth in search of a powerful energy source. The Justice League detects his arrival and confronts him. At first, they underestimate his power. Goku Black easily defeats some of the members, but then the League starts to work together and figure out his weaknesses. Another storyline could be that an evil entity has brought Goku Black to the DC Universe to cause chaos and the Justice League has to stop him while also dealing with the entity. Or perhaps Goku Black is tricked into thinking the Justice League are enemies of the Saiyans and he attacks them, only to later find out the truth and have to team up with them to fight a greater threat.
Well, a really cool storyline might be a time - travel fanfic where Percy and Jason are sent back in time to the era of the ancient Greek heroes. They have to work together to prevent a disaster that could change the course of history. Along the way, they learn about each other's powers and how to use them in different ways. There could also be a fanfic where they are pitted against each other in a gladiator - style arena, but with magical twists and turns that make it more than just a physical fight. Maybe they have to solve riddles or use their powers in creative ways to defeat their opponents.
One interesting storyline could be a Jedi and a Sangheili teaming up against a common enemy. They start off distrusting each other because of their different cultures and ways of fighting. But as they face challenges together, they learn to respect and rely on each other. Another could be a competition between a Jedi and a Sangheili to see who is the better warrior. This could involve a series of challenges and battles in different environments.