Yes, there might be. The Harry Potter and Sailor Moon fandoms are both large, and with the concept of crossover fanfiction, it's possible for someone to have written a story where Harry Potter is engaged to Sailor Saturn. You can try searching on popular fanfiction websites like Archive of Our Own or Fanfiction.net.
It could be really interesting. Maybe it would start with some kind of magical portal that brings Harry Potter to the Sailor Moon universe where he meets Sailor Saturn. Their relationship could develop as they face common enemies or try to understand each other's powers. The engagement could be a way to symbolize their union against a greater evil or just a result of their growing love.
A common theme is the clash and then the融合 (combination) of two different magical systems. Harry Potter has its own set of spells and magic, while Sailor Saturn has her unique powers from the Sailor Moon universe. Writers often explore how these two different kinds of magic interact.
Another possibility is a story set in a post - Voldemort era. Sailor Saturn appears in Hogwarts as a transfer student. Since she has such a strong and unique power, she catches Harry's attention. They start to train together, with Sailor Saturn teaching Harry some of her combat techniques and Harry teaching her about magic spells. This could lead to a deeper relationship between them as they face new threats to the wizarding world.
One great 'Harry Potter x Sailor Saturn' fanfic is 'The Meeting of Two Worlds'. It's really interesting as it combines the magic of Harry Potter's world with the unique powers of Sailor Saturn. The author does a wonderful job of creating a believable scenario where the two characters interact.
An interesting plot could be that a dark entity from another dimension is trying to collect powerful artifacts from both the Sailor Moon and Harry Potter universes. This entity targets the Silver Crystal and the Deathly Hallows. Sailor Saturn, Mamoru, and Harry have to work together to stop it. Harry uses his wizard skills, Mamoru his fencing and Tuxedo Mask powers, and Sailor Saturn her Sailor powers. Along the way, they face many challenges and have to learn to trust each other despite coming from different worlds.
Well, Sailor Saturn could use her powers to help Harry in a battle against dark wizards. Her powers over death and destruction could be a great asset in the wizarding world full of dangerous spells and dark creatures.
Another plot could involve a crossover with a science - fiction universe. Sailor Saturn might encounter advanced technologies and have to adapt. She could form an alliance with a group of space - faring adventurers to stop an intergalactic threat.
The story could start with Harry accidentally stumbling into the Sailor Moon universe. Mamoru, being the more experienced one, tries to help Harry understand this new world. Sailor Saturn, with her unique powers of destruction and rebirth, could hold the key to defeating a new threat that endangers both universes. For example, a powerful dark force that wants to use the combined power of the Deathly Hallows and the Silver Crystal. Their friendship and cooperation would be crucial in this battle, and along the way, they would learn from each other's strengths and weaknesses.
She is also a very determined character. So, when facing challenges in the crossover world, she should be shown as resolute in achieving her goals. Let's say she's in a crossover with a post - apocalyptic world. She would be focused on finding a way to restore order or save the survivors.