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.
It could be possible in fanfiction. Maybe Goku discovers a new type of wish - granting magic that is even more powerful than the Dragon Balls. Or perhaps he travels to a different dimension where time and death work differently. Fanfiction allows for creative interpretations. But in the official Dragon Ball story, there's no indication that such a thing could occur. The Saiyan race has its own limitations and the concept of death is pretty well - established in the series.
On a more magical level, reviving the dead is against the natural order of things in the wizarding world. This could attract unwanted attention from magical authorities or even dark forces. Some might see Harry's act as a dangerous precedent and try to either stop him or use his new - found power for their own evil purposes.
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.
On a personal level, Naruto would finally get the family he always wanted. But it might also bring some confusion and adjustment issues. His parents would be from a different time, and they'd need to adapt to the new world Naruto lives in. For example, they might be shocked by the new technology and the changes in ninja warfare.
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.
In some fanfictions, Harry might use extremely powerful and ancient magic. Maybe he discovers a long - lost spell during his adventures that has the power to reverse death. For example, he could find a spell hidden in a forgotten corner of the Hogwarts library that was created by a powerful wizard in the past but was deemed too dangerous to be used.
Well, Goku is a Saiyan, and Saiyans have a strong sense of family deep down, even if they don't always show it. When he meets his parents in this fanfiction, he could initially be in disbelief. But once he realizes it's real, he would embrace them wholeheartedly. He may even start to show off his strength and skills a bit, like a child showing off to his parents. And he would be excited to hear stories about their Saiyan race and how they ended up in the situations they did.
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.