Erza would also be a good pairing. She is a very disciplined and strong - willed character. Goku could learn from her discipline, and she could learn from Goku's carefree yet determined attitude. They could fight side by side in battles and protect each other.
It could also be a symbol of a new power - up. In the Dragon Ball series, new physical changes often lead to increased power. So, getting his tail back might mean that Goku is about to reach a new level of strength. Maybe he'll be able to transform into a new form that is related to his Saiyan heritage and the presence of the tail. This could give him an edge in future battles against stronger opponents.
Well, in a lot of fanfictions, the story might rewrite the rules of the Dragon Ball universe. So Goku could keep his tail simply because the author wants to explore different storylines with a tailed Goku. Maybe the author thinks it adds more to his character's appearance and abilities, like being able to use it as an extra limb in combat or for balance during his super - fast flying.
Well, one possible plot could be that Goku somehow ends up in the Fairy Tail world. He might initially be confused by the magic there, but then start to learn about it. He could team up with Natsu and the gang to fight against a powerful dark wizard. Goku's strength and his unique fighting skills like the Kamehameha could be a great asset to Fairy Tail in battles.
Well, Goku's powers are extremely powerful. In Fairy Tail, magic is the main power source. Goku's ki-based abilities could be seen as a super-advanced form of magic. His super strength, speed, and energy blasts could make him a formidable ally or a terrifying foe. He could easily take on some of the stronger dark guilds by himself. Also, his ability to constantly grow stronger through training would be a unique aspect compared to the mages in Fairy Tail who mainly rely on learning new spells.
Another possibility is that a new form of training or power - up in the fanfiction is related to the tail. So Goku is made to keep it for his growth and development as a fighter.
One possible plot could be that Goku somehow gets transported to the Fairy Tail world. He might initially be confused but quickly adapt due to his adventurous nature. He could team up with Natsu and the others to fight a common enemy, like a powerful dark guild that has a magic that can rival Goku's strength. Their combined powers would make for an epic showdown.
In some fanfictions, it might grow back due to magical or alien forces. For example, a powerful wizard in the fanfic universe could use a spell to regrow it. Another way could be through some sort of advanced medical technology created by the Saiyans or their allies.
Well, one unique aspect is his power level compared to the Fairy Tail characters. Goku Black is extremely powerful, so in fanfictions, he can either completely overpower the Fairy Tail mages or he might teach them new ways to increase their strength. For example, his ki - based attacks are a completely different form of energy than the magic in Fairy Tail.
There could be a fanfiction where Goku is a long - lost relative of a powerful Fairy Tail wizard. In this story, he discovers his family ties while also having to face off against a common enemy that threatens both his Saiyan heritage and the Fairy Tail guild. He would bring his Saiyan strength and tenacity to the battles in the Fairy Tail world.