One possible plot could be that there is a rift in the multiverse that brings Goku and Luffy together in a strange new world. They initially misunderstand each other and start to fight. As the fight progresses, they both realize that there is a greater threat at hand that is causing the rift. So, they team up, with Goku teaching Luffy some of his combat techniques and Luffy showing Goku the value of friendship and loyalty in a different way.
The plot could start with Goku being accidentally transported to the One Piece world. Luffy mistakes Goku for a new and powerful enemy. Goku, in his Ultra Instinct state, is just trying to find a way back home. Their first encounter is a battle where Luffy is shocked by Goku's speed and power. However, as the fight continues, Luffy's tenacity and unique abilities start to surprise Goku. Then, they both discover that there is a dark force in the One Piece world that is connected to a similar threat in Goku's universe. So, they decide to put aside their differences and work together to defeat this common enemy.
Goku in Ultra Instinct mode is extremely powerful. He has superhuman speed, strength, and can dodge attacks effortlessly. However, Luffy also has his own set of amazing abilities like his Gear Fourth form which gives him enhanced speed and power. But I think Goku would win because his combat skills and power level are just on a different scale compared to Luffy.
One idea could be a crossover with the Naruto universe. Goku with his Ultra Instinct could face off against Naruto in his strongest form. They might start as rivals but then realize they have a common enemy and team up. The story could explore how their different fighting styles and powers interact.
Maybe Goku gets mixed up in the events of the Injustice storyline. He could initially be confused by the alternate version of Superman who has gone bad. Goku, with his pure heart and Ultra Instinct, might try to bring the corrupted Superman back to the light. This could lead to some intense battles with the Injustice League as Goku tries to set things right.
One possible plot twist could be Goku traveling back in time to train with his past self. This would not only enhance his own power but also change the course of his past battles. For example, he could teach his past self Ultra Instinct earlier, leading to different outcomes against foes like Frieza or Cell.
A possible plot is a crossover with the Naruto universe. Goku in Ultra Instinct mode could be mistaken for some sort of super - powered ninja. He might end up training with the Hokage or getting involved in the Akatsuki's plans. Since his Ultra Instinct gives him such incredible reflexes and power, he could easily hold his own against the strongest ninjas like Naruto in his Six Paths Sage Mode or Sasuke with his Rinnegan. It would be a clash of two very different power systems.
A really interesting storyline could be that an ancient prophecy in the Fairy Tail world foretells the arrival of a being with ultimate power like Goku. The Fairy Tail guild is divided on whether to welcome him or be afraid. Meanwhile, there are other forces that want to use Goku for their own evil purposes. The story could revolve around Goku trying to figure out his place in this new world, while also having to deal with the various threats and the complex relationships within the Fairy Tail guild.
Well, with Ultra Instinct, Goku would be able to react extremely quickly to the DC heroes' powers. For instance, when facing Flash, who is super fast, Goku in Ultra Instinct might be able to match his speed or even anticipate his moves better. This could lead to a mutual respect between them as they test each other's limits.
First, understand the characters well. Know Goku's Ultra Instinct powers, his personality, his goals. Then, when choosing a crossover universe, pick one that has elements that can contrast or complement Goku's world. For example, if it's a magic - based universe, how would Goku's physical powers interact with magic? Keep the story focused on the main characters and their development throughout the crossover.
Yes, it is possible. In fanfiction, writers have the creative freedom to reimagine the origin of characters. Goku being born with Ultra Instinct can add a whole new layer to his character development. It could mean that his power growth would be different from the original story, perhaps faster or with different challenges along the way.
You could write a fanfiction where Goku teaches others about Ultra Instinct. Maybe he tries to pass on this power to Gohan or Trunks. This would involve Goku having to understand the power more deeply himself in order to teach it, and it would also be a great way to develop the other characters.