A third idea could be that the nekos in the Ranma universe are somehow related to the cats in the Sailor Moon world. Maybe Luna could sense something different about Ranma when he's in his cat form (if he has one in this crossover). They could team up to find out more about this connection and in the process, discover a new threat that requires both the power of the Sailor Scouts and Ranma's unique abilities to defeat.
Another interesting concept is a love triangle. Suppose Ranma falls for Sailor Moon, but Tuxedo Mask is already in the picture. This could lead to a lot of comical and tense situations. Ranma might try to outdo Tuxedo Mask in protecting Sailor Moon, leading to some friendly rivalry. Also, his curse could add an extra layer of complication. What if he turns into a girl when he's around Sailor Moon at an inconvenient time?
One idea could be that Ranma accidentally stumbles into the world of Sailor Moon. Maybe he gets caught up in a fight against a youma and the Sailor Scouts mistake him for an enemy at first. But then they realize he has his own unique fighting skills and they team up to defeat the common foe.
One idea could be that Ranma somehow gets involved in the fight against the Dark Kingdom in the Sailor Moon universe. Maybe his curse could be related to the magic there. For example, his female form could be mistaken for a new Sailor Scout by the bad guys.
Maybe there could be a villain that is causing problems in both worlds. The Sailor Scouts and Ranma could team up to fight this villain. Ranma's martial arts skills combined with the Sailor Scouts' magical powers would make for an interesting battle. They might have to learn to work together despite their very different fighting styles and personalities.
One idea could be a crossover with the Harry Potter universe. Sailor Moon and her friends could attend Hogwarts and have to deal with both magical threats from the wizarding world and the dark forces they usually face. For example, they might help the students at Hogwarts fight off a new kind of dark magic that has similarities to the dark energy they combat in their own world.
One idea could be that the Sailor Scouts somehow end up in the SVTFOE universe. They could team up with Star and Marco to fight a common enemy. Maybe an evil force that combines the darkness from both universes.
Another fanfic concept is that there's a threat in Gotham that has a connection to the Moon Kingdom's past. Sailor Moon travels to Gotham to investigate. There, she meets Batman, who is initially skeptical of her magical powers. But as they start to work together, they realize that their skills are complementary. For example, Sailor Moon's ability to heal could be useful for Batman's injured allies, and Batman's detective skills could help Sailor Moon figure out the mystery behind the new threat.
One idea could be a story where Goku and Sailor Moon team up to fight a common enemy. Maybe a powerful demon that is threatening both their worlds. They could learn from each other's fighting styles, like Goku showing Sailor Moon how to better use her physical strength and Sailor Moon teaching Goku about the power of friendship and love in combat.
In a crossover, the characters might interact through a shared mission. Let's say there's a powerful villain that is causing chaos in both the Ranma and Sailor Moon universes. Ranma and the Sailor Scouts would have to put aside their differences and work together. Akane from Ranma's world could be jealous of Ranma's closeness to Sailor Moon, which would create some drama. Meanwhile, Sailor Mars might be suspicious of Ranma at first, thinking he could be a new enemy, but as they work together, she realizes he's an ally. This would lead to a development of their relationships.
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.
A fem Ranma crossover fanfic could involve crossing over with a historical drama. Fem Ranma could be transported to a different historical period, like feudal Japan or Victorian England. In feudal Japan, she could use her martial arts to become a respected warrior or a protector of a village. In Victorian England, she might have to navigate the strict social rules while still being able to show off her fighting abilities. Another interesting crossover could be with a superhero universe. Fem Ranma could be a new superhero or an anti - hero, using her martial arts and unique situation (being male - female - changing) to fight crime and deal with other superheroes and villains.