One common theme could be adventure. Since it's set in an 'underground' setting, there might be elements of exploring the unknown, facing challenges and obstacles. Another theme might be the relationship between rex and the devilartemis characters, which could involve friendship, rivalry or even some form of cooperation.
One idea could be a crossover with the Dragon Ball Super universe and the DC Comics universe. Zamasu could team up with Darkseid, believing they share a similar view of bringing 'order' to their respective realms. Another idea is a crossover with the Naruto universe, where Zamasu tries to impose his will on the ninja world, facing off against powerful shinobi like Naruto and Sasuke. Or perhaps a crossover with the Star Wars universe, with Zamasu as a Sith - like figure, trying to gain control over the Force.
One interesting Zamasu fanfic could be about his origin story in more detail. It might explore his early days in the Kaioshin realm, his training, and how his ideology started to form.
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.
Well, I'm not that familiar with specific 'rex in the underground devilartemis fanfiction' stories. But you could try searching on popular fanfiction websites like Archive of Our Own. They usually have a wide variety of fanfictions and you might be able to find some good ones related to this there.
The idea of an alternate universe is also very common. Since Zamasu is from a different franchise, fanfictions like to create a new world where both Naruto and Zamasu co - exist. This allows for new character relationships to be formed, new storylines to be developed, and for the writers to be really creative with the setting and the overall plot.
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.