Another plot could be the harem members helping Goku in a new and unexpected way. Maybe they use their unique skills or knowledge to assist him during a tough battle or in solving a mystery related to the Dragon Balls. Since they come from different backgrounds, their combined efforts can create really engaging storylines.
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.
Bulma could be one of the characters in the harem. She has a long - standing relationship with Goku and her intelligence could complement Goku's power. Another could be Chi - Chi, his wife. Her fierce loyalty and determination would make her an interesting part of the harem.
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.
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 Godzilla protecting his harem from other kaiju threats. He uses his powerful atomic breath and strength to keep his mates safe.
One storyline could be Goku using his smartness to outwit a new and powerful enemy. For example, instead of relying solely on his brute strength, he analyzes the enemy's weaknesses and devises a clever plan. Maybe he figures out a pattern in the enemy's attacks and uses it to his advantage, like dodging at the exact right moments and then counter - attacking with precise strikes.
One idea could be Goku from Dragon Ball crossing over into a world like Fairy Tail and having a harem of the female mages there. Maybe he helps them in battles and gradually forms relationships with them.
One idea could be that Goku is transported to a different universe where he meets various female characters from different anime. They are all intrigued by his power and his pure-hearted nature, and a harem dynamic starts to form as they all want to be around him and train with him.
In some fanfictions, a possible plot is that Android 21 discovers a way to transfer some of Goku Black's power into herself, creating a new form for her. This form has a mix of her android features and Goku Black's dark energy, leading to a whole new set of adventures as she tries to control this power.