One possibility in fanfiction is that Goku comes across a special energy source. This energy source could be something like a rare crystal or a unique form of ki. When he absorbs or is exposed to this energy, it triggers the regrowth of his tail. This could also be tied to a prophecy or an ancient Saiyan legend that the author has created within the fanfiction.
Well, it could be that Goku's body has some latent genetic ability. In the fanfiction world, the author might decide that when Goku reaches a certain power level or goes through a particular transformation, his tail grows back as part of his Saiyan heritage reasserting itself. Maybe it's related to the Saiyan's natural resilience and ability to heal and regenerate.
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.
In the Dragon Ball universe, there are many strange and wonderful things. King Kai might have access to some rare or magical items. He could use one of these items to help Goku's tail grow back. For example, there could be a special fruit on his planet that has regenerative properties. When Goku consumes it or King Kai uses it in some other way, it triggers the growth of the tail.
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.
The tail - growing - back process might have a profound impact on Saiyan culture. In their society, the tail has always been a part of their physical identity. If it grows back, it could change the way they view their own strength. For example, they might develop new fighting styles that incorporate the use of the tail more effectively. And it could also influence their mating rituals as the tail is often associated with Saiyan's primal instincts.
Perhaps it was the result of a special training or a new form of energy absorption. Gohan might have been exposed to a new type of ki or energy that stimulated the regrowth of his tail. For example, if he was in a place with a high concentration of a certain type of energy that interacted with his Saiyan cells in a way that caused his tail to grow back.
One scenario could be that Goku enters a special realm where time and space work differently. In this realm, the laws of biology are altered, and his tail simply regrows. Another might be that an enemy's attack, instead of harming him further, accidentally triggers a hidden regenerative ability in Goku that causes the tail to grow back.
He might be surprised at first. After all, having his tail back is a big change. He could start to explore the new sensations and abilities that come with it, like better balance perhaps.
Goku might become more in touch with his Saiyan heritage. His tail was a part of what made him a Saiyan originally, so getting it back could make him more aware of his roots. He could become more aggressive in battle, like how Saiyans are known to be.
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.
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.