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.
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 Goku never lost his tail, his powers and abilities might have developed differently. Maybe he would have retained some primal monkey-like traits that could have given him unique advantages or disadvantages in battles.
For Goku's relationship with the other Z - Fighters, it could be a mixed bag. Some, like Piccolo, might see the tail as a potential weakness that enemies could target. So he might be constantly on Goku to protect it. On the other hand, Yamcha might be jealous that Goku has this unique feature that makes him stand out even more. However, overall, Goku's tail could also be a unifying factor. It could remind them all of Goku's alien origin and make them more aware of the vastness of the universe and the different species within it, which could strengthen their bond as a group.
One interesting plot could be that Goku's tail gives him a new power. Since it's always been a unique part of him, perhaps it allows him to communicate with a certain race of creatures that are only attuned to the energy his tail emits. Another plot could be that he has to protect his tail from an enemy who wants to use it for some dark magic. The enemy believes that Goku's tail holds the key to unlocking a powerful spell, so Goku has to constantly be on guard while still going about his normal adventures.
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.
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, Goku's powers are mainly based on ki manipulation, which is somewhat similar to magic in the Fairy Tail world. If he joins Fairy Tail, he could probably learn to combine his ki with the magic of Fairy Tail. For example, he might be able to infuse his ki into Natsu's fire magic to make it even more powerful. Also, his teleportation technique, like Instant Transmission, could be a great asset during battles and rescue missions in the Fairy Tail universe.
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.