One plotline could be that Goku's tail is the key to unlocking a new form. Since it's such an important part of his Saiyan heritage, it could be tied to some ancient Saiyan power. Maybe he has to go on a journey to discover how to fully utilize his tail and in the process, he unlocks a form that's even more powerful than Super Saiyan. Another could be that his tail is cursed. Every time he uses it in combat, it drains his energy instead of helping him, and he has to find a way to break the curse.
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.
His tail might enhance his combat abilities in a few ways. Since Goku is part Saiyan and Saiyans are known for their agility, the tail could be an extension of that. He could potentially use it to sense movements around him, much like how some animals use their tails for that purpose. In combat, this could give him an edge in predicting attacks and reacting faster. Also, he could use it for quick strikes, like a whip, adding an unexpected element to his fighting style.
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.
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.
If DBZ Goku were in Fairy Tail, his sense of justice would fit right in. He could form strong bonds with the Fairy Tail members. His training regime might even influence some of the wizards to train harder. And when it comes to battles, his ability to sense energy would give him an edge over many foes. He could also learn some of the Fairy Tail magic techniques and combine them with his own powers in really interesting ways.
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.