Well, it could be that an evil villain from another dimension kidnaps Sailor Moon to use her powers for their own dark purposes. Maybe they want to drain her energy to open a portal to a world of chaos.
One possible plot is that an alien race, not knowing the true nature of Sailor Moon, kidnaps her thinking she's a powerful weapon. They then try to study her and in the process, she has to find a way to escape and also teach them about love and friendship which are her powers' source.
Perhaps in the fanfiction, a jealous rival from within the Sailor Scouts' group gets influenced by a dark force and kidnaps Sailor Moon out of envy. This could lead to a story of betrayal and redemption among the Scouts.
The Sailor Guardians could start by using their powers together. Sailor Moon could use her Moon Healing Escalation to try and sense any dark energy around where Michiru was last seen. They could also split up and search different areas. Maybe Neptune's own powers could be used in some way to call out to her or track her down. Once they find the location, they would fight off the kidnappers using their combined attacks.
Well, one possible plot could be that the Sailor Scouts are transported to the Pokemon world. They might team up with some Pokemon trainers to fight against a common enemy. Maybe an evil organization is trying to use both the power of the Dark Kingdom from Sailor Moon and some powerful, dark Pokemon. The Sailor Scouts' magical powers could interact in interesting ways with the Pokemon's abilities.
One possible plot could be that Voldemort teams up with a Sailor Moon villain. Harry and the Sailor Scouts then have to join forces to defeat them. They would bring together their different magical abilities, like Harry's spells and the Scouts' transformation powers.
Perhaps in the fanfiction, Naruto and Sailor Moon are siblings in a world where both ninja and magical girl powers exist side by side. Naruto could teach Sailor Moon some of his combat techniques, like how to use a kunai effectively. In return, Sailor Moon could show Naruto the power of her transformation and how to use her tiara as a weapon. Their relationship could be centered around protecting their city or world from a common threat that combines the dark forces from both of their original stories.
Maybe it's a group of anti - Sailor Scouts. They are people who oppose the Sailor Scouts' mission of protecting the world and think that by kidnapping Sailor Moon, they can stop the Scouts' activities. They might be under the influence of an ancient evil.
One possible reason could be that Esmeraude wants to use Haruka as a bargaining chip. Maybe she plans to exchange Haruka for something valuable, like a powerful crystal or a secret from the Sailor Guardians. Haruka is a strong and important character, so she would be a valuable hostage.
If Michiru was kidnapped in a fanfiction, the other Sailor Guardians would surely spring into action. They would first try to gather any clues about her whereabouts. Mercury would use her intelligence and technology skills to analyze the situation. Mars might use her psychic powers to sense any danger or the location of Michiru. They would then form a plan to rescue her, facing whatever challenges or enemies are behind the kidnapping.
One possible interesting plot could be that Naruto and Sailor Moon somehow end up in the same dimension due to a strange magical rift. Naruto, with his ninja skills, and Sailor Moon, with her magical powers, have to team up to fight a new common enemy that is causing chaos in this merged world. They start off not understanding each other's abilities but gradually learn to combine their powers for greater effect.
Perhaps in the fanfiction, female Harry Potter has a magical encounter that transports her to the Sailor Moon universe. There, she learns about the power of the Sailor Scouts and their way of fighting evil. She then brings some of those powers or ideas back to the wizarding world. For example, the concept of teamwork in a more Sailor Moon - like way, where each person has a specific role and power that combines for a greater good. This could change how she approaches her relationships with Ron and Hermione and how they all fight together.
One interesting plot could be that the Sailor Guardians are on a mission to a world similar to Naruto's. There, they meet Naruto and his friends. Naruto is initially confused by their strange powers but quickly realizes they can help fight a common enemy. Maybe an evil force from the Sailor Moon universe has teamed up with Orochimaru. The Sailor Guardians' magic combined with Naruto's ninja skills could make for some epic battle scenes.