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.
Power comparison is a common theme. For example, when Goku crosses into the Marvel universe, fans often like to see how his Saiyan powers stack up against the likes of Thor or Hulk. Another theme is culture clash. When Goku enters a different fictional world like the world of Bleach, his Saiyan values and the way of life in Soul Society are very different, and this contrast is explored in the fanfiction.
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.
Bulma could be involved. Since she has a relationship with Vegeta and is friends with Goku, she might try to mediate between them when Goku snaps at Vegeta. She could be the voice of reason, trying to calm both of them down and make them see each other's point of view.
It often starts with a shock or surprise at the mpreg situation. Then, they gradually learn to rely on each other more. For example, Vegeta might start to show a more caring side that he usually hides, and Goku might become more responsible. They'll have to communicate more, sharing their fears and hopes about the future with the baby.
Yes, in the vast world of fanfiction, there are all kinds of creative and out - of - the - box stories. Some fans like to explore unusual relationships and scenarios, so it's possible that there is such a fanfiction out there. However, it goes against the original canon of the Dragon Ball series in many ways.