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.
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.
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.
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.
In a fanfiction scenario where Goku gets his tail back, it might be due to a genetic experiment gone right. Some scientist in the Dragon Ball world, either good or bad, might be working on Saiyan genetics. They accidentally (or on purpose) trigger the regrowth of Goku's tail. Once he has his tail back, it could change his relationship with other characters. His son Gohan might be really interested in his dad having a tail again. Also, Goku could use his tail in combat in new and unexpected ways, like using it to grapple opponents from a distance or as a distraction during a fight.
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.
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.