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.
It can be. There are fans who love to explore different relationship dynamics for Goku in a harem - like setting in fanfiction. But the 'lemon' part might put off some readers. Some communities have strict rules against such content. So in a more open - minded and lenient fanfiction community, it could be relatively popular, but not so much in a conservative one.
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.
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.
A great 'Goku Harem Fanfiction' is 'Goku and His Female Allies'. This story delves into how Goku's relationships with the female characters in his harem develop over time. It includes elements of adventure as Goku and his harem members face various threats together. The fanfiction also explores how the different personalities in the harem interact with each other and with Goku.
In a crossover with Sailor Moon, Sailor Venus could be part of the harem. She is a strong fighter like Goku in her own way, and their different powers and personalities could create interesting dynamics.
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.
Chi - Chi is often a key character. As Goku's wife in the original series, she has a significant role in fanfictions too. She can be shown in different ways, sometimes being more protective or sometimes more involved in Goku's adventures with his harem - like group.
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.