One common theme is the exploration of Goku's relationships outside of his usual fighting and training. Another is the development of the female characters in the harem, showing their unique personalities and how they interact with Goku.
One popular 'Goku Harem Fanfic' story could be the one where Goku's different relationships with various female characters from the Dragon Ball universe are explored in a romantic context. For example, some might focus on his interactions with Chi - Chi, Bulma, and Android 18 all at once. Another might be where new female characters are introduced into the story specifically for the harem concept.
Well, 'Goku's Galactic Harem' is also quite known. In this fanfic, Goku travels across the galaxy and gathers female allies who gradually become part of his harem. The stories often include epic battles against powerful foes, and the harem members use their unique skills to support Goku. It's interesting to see how Goku's character is adapted to fit into a harem - themed story, as he is usually so focused on fighting and training in the original series.
Love and redemption can also be common themes. Sometimes the harem members help Goku Black to find a new path or redeem himself through their love. It adds an interesting layer to his character.
Often, in 'Goku Harem Fanfic', Goku becomes more aware of his romantic side. He might be a bit more in tune with the emotions of the female characters around him compared to his original portrayal in the Dragon Ball series, where he was mostly focused on fighting and training.
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.
One possible interesting plot could be Goku, in his godlike form, being transported to different universes where he meets various female characters from different shows or stories. He then has to form alliances with them to fight a common evil. As they spend time together, romantic feelings start to develop among them, creating a harem dynamic.
One idea could be a story where Goku is transported to the Bleach universe and somehow ends up attracting a harem of female characters from Bleach. For example, Orihime could be intrigued by Goku's incredible strength and kind heart, and gradually develop feelings for him. Another could be that Goku saves Rukia from a powerful enemy in the Bleach world, and as a result, she and other female Soul Reapers start to follow Goku around, leading to a harem - like situation.
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.