One idea could be that the OC is a new senshi from a far - off planet who arrives on Earth just as a new threat emerges. The Sailor Scouts are initially suspicious but soon realize they need all the help they can get.
Perhaps the OC has a special power that is linked to the moon in a different way than Sailor Moon's. For example, the OC can communicate with the spirits of the moon's past inhabitants and they provide crucial information to defeat an enemy. This would add a new layer of mystery and lore to the Sailor Moon universe.
Another interesting concept is an OC who is a villain at first but later becomes an ally. Maybe they were brainwashed by the real villains. Their journey from the dark side to the good side can be full of drama, self - discovery, and interesting interactions with the Sailor Guardians.
One interesting plot idea could be a time - travel scenario where Sailor Moon goes back in time to prevent a major event that led to the rise of a powerful enemy. Another could be a crossover with another popular anime, like having Sailor Moon meet Naruto and they team up to fight a new threat.
One idea could be that Usagi, being the kind-hearted leader she is, gradually forms a harem of characters who are drawn to her pure heart and strong sense of justice. For example, the various Sailor Scouts could develop deeper feelings for her beyond just friendship, leading to a complex web of relationships.
One idea could be that Naruto somehow gets transported to the Sailor Moon universe. In this new world, the Sailor Scouts initially see him as an oddity but gradually come to respect his ninja skills. As for the harem aspect, Naruto's kind heart and unique abilities attract the Scouts, leading to a series of adventures where they all have to work together to protect both their worlds from a common threat. Maybe a new villain emerges who combines the dark jutsu of Naruto's world and the dark magic from the Sailor Moon world.
A great concept for a Sailor Moon OC fanfic is an OC that has a special connection to the moon but not in the same way as Sailor Moon. This OC could have been created by an ancient civilization on the moon to be a protector. She might have different abilities like being able to control lunar phases for various effects, such as using a new moon to create a shield or a full moon to enhance her strength. Or an OC who is an artist, and she uses her art to fight the evil. Every time she draws something, it comes to life to battle the bad guys.
Well, in Sailor Moon dark fanfiction, a cool plot could be the Sailor Guardians facing an enemy that can manipulate their worst fears. This would make them question their own abilities and loyalties. Also, a story where the power source of the Sailor Guardians is somehow tainted, turning their powers dark and making them struggle to control it. Or perhaps a plot where a new dark Sailor Guardian emerges, with powers equal to or greater than Sailor Moon's, causing chaos in the magical world.
One idea could be that the characters from RWBY end up in the Sailor Moon universe and have to learn the ways of the Sailor Scouts. For example, Ruby could be intrigued by Sailor Moon's transformation and try to adapt her Crescent Rose to work in that magical world.
An interesting plot could be that Goku and Sailor Moon meet during a multiverse tournament where representatives from different anime universes are competing. They initially don't get along because of their different fighting styles and beliefs, but as the tournament progresses, they realize they can learn from each other. Together, they face off against a cheating opponent who is using unfair tactics to win. Through their combined efforts, they not only defeat the opponent but also learn to respect each other's worlds and ways of fighting.
One idea could be a story where a new Sailor Guardian emerges from a different dimension and has a unique power that can either help or hinder the fight against the Black Moon Clan. For example, she might be able to manipulate time in a different way than Sailor Pluto. Another plot could involve a member of the Black Moon Clan who has a change of heart and decides to secretly assist the Sailor Guardians. This would create a lot of internal conflict within the clan.
Well, a cool plot might be that the Dragon Balls are stolen by an evil force from the Sailor Moon universe. So, the Z Fighters have to travel to that universe to retrieve them. Along the way, they meet the Sailor Scouts and realize they need to work together. The Sailor Scouts could also learn about the power of the Dragon Balls and how they can be used for good. They might even help the Z Fighters in their search by using their unique powers to track down the stolen balls.