You can start by checking out your local bookstore. Go to the sections for urban fantasy and romance, and look for titles that seem to combine both elements. Often, the covers or blurbs will give you an indication.
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.
Another great one is 'The Fever Series' by Karen Marie Moning. It has a unique blend of urban fantasy with a strong romantic sub - plot. The story is set in Dublin and involves a lot of mystery, ancient magic, and a passionate relationship between the main characters.
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.
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.