One could also write a fanfiction about a school exchange program. Hogwarts students could visit the Sailor Moon Cosmos schools and vice versa. The cultural shock would be huge. For example, the Hogwarts students would be used to using wands, while the Sailor Moon Cosmos students might use their transformation powers and magical items in different ways. There could be some friendly competitions between the students of both universes, like a magic - using race or a knowledge - based quiz about different magic systems.
In a Harry Potter and Sailor Moon Cosmos fanfiction, the characters' personalities would create some really engaging scenarios. Hermione would probably be fascinated by the technology and different planets in the Sailor Moon universe. She might try to analyze how their society works compared to the wizarding world. And Sailor Moon herself, with her kind and brave nature, could form a great friendship with Harry. Their combined strength could be used to fight against a new threat that somehow affects both universes.
The Sailor Stars in the Sailor Moon manga are not transgender. The manga portrays their genders consistently without any indication of them being transgender.
Sure, the Sailor Moon manga is finished. It wrapped up nicely, giving closure to all the characters and their adventures. But you can still enjoy re-reading it or exploring related merchandise and media.
The number of Sailor Moon manga varies. There are the main series, along with special editions and adaptations. But as of now, I'd say there are at least a dozen or so.