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.
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.
Well, in such a fanfiction, the art styles could be blended in an interesting way. The Sailor Scouts could have a new look inspired by Batman's aesthetic, like darker colored fuku with bat - like symbols. And Batman could get a touch of Sailor Moon's magic, perhaps his utility belt having some magical items. The story could explore how two very different universes collide, and how the characters grow from the experience.
Friendship is a big theme too. Characters from different universes in Sailor Moon crossover fanfic often become friends and learn from each other. They support each other in battles and in personal growth. Also, the theme of saving multiple universes is quite common. Since it's a crossover, there are usually threats that endanger both the Sailor Moon universe and the other crossed - over universe.
Maybe in the fanfic, Batman's rogues gallery somehow teams up with some of the Avengers' villains. This forces Batman and the Avengers to work together. Batman could introduce his unique gadgets and combat style to the Avengers, while they could show him their superpowers up close. For example, Iron Man could be intrigued by Batman's utility belt and try to create his own version of some of the gadgets.
Sailor Moon would probably be really curious about Batman. She'd be constantly asking him questions about his gadgets and his mission. Batman, being the brooding type, might be a bit annoyed at first but would soon realize her good heart. He'd start to respect her powers and they'd form an unlikely alliance.
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.
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.
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.