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.
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.
Well, it's really hard to say for sure. Ultra Instinct Goku is extremely powerful with his god - like reflexes and insane combat skills. But Fairy Tail characters also have some very strong magic. If we consider Goku's speed and strength, he might have an edge. However, some Fairy Tail mages could potentially use powerful spells to counter him. So it could go either way depending on how the fanfiction is written.
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.
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.
Goku definitely masters Ultra Instinct in the manga. This transformation gives him a huge boost in power and allows him to take on even stronger opponents.