Some dedicated Dragon Ball fan communities on social media platforms might also be a good source. For example, certain groups on Facebook or Reddit where fans discuss and share their fanfiction works. However, the availability of exactly this pairing might vary. You may need to do some searching within those communities.
It gives a chance for creativity in exploring gender - swapped or alternate versions of the characters. Fanfiction writers can play with the personalities and backstories of these characters in ways that the original series might not have explored. For example, they can write about how the female Vegito might approach a relationship with Gogeta differently than a male Vegito would, adding new layers to the story.
A common theme is power - seeking. They are often depicted as constantly striving for more power and ruling over the universe. Another theme is their unique bond, whether it's a friendship or a more sinister partnership.
One popular 'goku black x zamasu fanfiction' is 'The Dark Alliance'. It delves deep into their partnership and the chaos they create in the universe. Another interesting one could be 'The Merged Power', which explores what would happen if they combined their powers in different ways.
Zamasu meets his end in the manga because he underestimates the strength and strategy of Goku and Vegeta. They exploit his weaknesses and deliver the final blow that finishes him off.
If Vegito never unfused, it might mean that the enemies he faced were simply not strong enough to force the separation. This could imply that the story's difficulty level was lower than originally thought. Or, it could be that some external factor prevented the unfusing, creating a whole new set of plot twists and complications.
One common theme is the clash of ideologies. Naruto has his values of friendship and protecting the village, while Zamasu has his own extreme views. So, this often leads to interesting conflicts in the fanfics.
Redemption could also be a theme. Although Zamasu in the original was quite villainous, a female Zamasu in fanfic might be on a journey of self - discovery and redemption. She could start as a bad character but gradually change due to certain events or interactions in the story.
She might be depicted as having a complex personality. On one hand, she could have a cold and calculating side due to her divine or self - proclaimed superior nature. On the other hand, there could be a hidden vulnerability that only the male reader gets to discover. This duality can make for an interesting exploration in the fanfic.