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, in some fanfiction, Goku keeping his tail could lead to him having a different form. Maybe the tail is the catalyst for a new, more primal form that combines his Saiyan heritage in a more raw way. He might struggle to control this form at first, but as he learns, he becomes even more powerful. Also, it could be a story where Goku's tail is a link to his past, and he starts having visions of his Saiyan ancestors through it, which guide him on a new quest to discover more about his roots.
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.
Perhaps in the fanfiction, Goku's love for his parents is so strong that it creates a new type of energy. This energy is noticed by a powerful and benevolent entity who offers Goku a deal. If Goku can prove that he will use his power for good and protect the universe even better with his parents alive, then he can revive them. Goku then has to go on a journey to show his worthiness, facing off against old and new enemies along the way.
Perhaps the reincarnated character has to face a situation where they are not fully accepted as Goku by his friends and family. They then have to prove themselves not just in terms of strength but also in terms of character, and in the process, they form new and deeper relationships with the Z - fighters.
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.
One interesting plot could be Goku and Fem Frieza having to team up against a new, powerful enemy. Maybe this enemy is threatening both of their planets or has some power that neither can defeat alone. Another plot could be Fem Frieza falling in love with Goku and trying to change her evil ways to be with him, but struggling with her past and her reputation among the villains.
Some fanfictions might have a plot where Goku and Naruto are in a training - like fight. They exchange techniques and learn from each other. For example, Goku could teach Naruto about ki control, and Naruto could show Goku how to use chakra in a more strategic way. Their fight could be less about winning and more about mutual growth and respect for each other's abilities.
In some 'oc son of goku fanfiction', a common plot is the OC's struggle with his identity. He is the son of Goku, a great hero, but he wants to make a name for himself. So he might embark on a quest to find a new form or a new fighting style that is all his own. Also, there could be a plot where he has to deal with the jealousy of other Saiyans who feel he gets special treatment due to his father.
One interesting plot could be Goku having to balance his training and his relationships with the various characters in his harem. For example, he might have to deal with the jealousy among the girls when he spends more time with one over the others.
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.
The most interesting aspect is probably the new power - ups. Since he still has his tail, there could be new forms related to it. Maybe a combination of his normal Super Saiyan forms and his ape - like abilities. It's exciting to think about how this would look in battles.