One idea could be a crossover where Ayato Sakamaki, from the Diabolik Lovers universe, and Usagi Tsukino, from Sailor Moon, meet in a modern city. Ayato is intrigued by Usagi's positive energy and Usagi is both scared and curious about Ayato's dark allure. They could form an unlikely friendship and help each other face their respective challenges.
A significant theme is the idea of saving the new world she enters. Just as she protects Earth in the Sailor Moon universe, in crossovers she often finds herself in a position to help save the world she has crossed over into. This could involve fighting against new villains or resolving conflicts within that world. It shows her as a hero in different contexts and how her values are applied in various fictional settings.
One great 'usagi tsukino crossover fanfiction' is 'Usagi Tsukino in the World of Harry Potter'. It's really interesting to see how Usagi's magical girl powers interact with the magic in the wizarding world. Another one could be 'Usagi Tsukino Meets the Avengers'. The idea of her joining forces with superheroes is full of potential for exciting storylines.
Love is a very common theme. Since Usagi and Mamoru have a strong romantic connection in the original story, fanfictions often expand on their love, like how they overcome obstacles to be together.
One great 'usagi x mamoru fanfic' is 'A Starry Encounter'. It beautifully portrays their relationship with vivid descriptions of their interactions. The author does an amazing job of capturing their chemistry.
In another scenario, Usagi and Mamoru could be at a ball. They dance gracefully together, lost in each other's eyes. After the ball, they take a stroll along the beach. The sound of the waves crashing against the shore sets a romantic mood. They hold hands tightly and exchange sweet nothings, reaffirming their love for each other.
You can try fanfiction platforms like Archive of Our Own (AO3). It has a vast collection of fanfictions on various topics, including those related to Usagi Tsukino and Haruka Tenoh. Just use the search function and type in their names to find relevant stories.
One good Mamoru and Usagi fanfiction could be 'Moonlight Encounter'. It beautifully captures their relationship in a new light. The author describes their interactions with great detail, making it a very engaging read.
Usagi and Mamoru could have a story where they volunteer together at an animal shelter. They bond over their love for the cute animals. One day, a little puppy gets sick and they work together to nurse it back to health. This shared experience makes their relationship stronger, and they realize how well they work as a team, which deepens their love.