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.
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?
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.
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.
The female characters in Ranma 1/2, like Akane, might be a bit jealous of the Sailor Scouts at first. They're so beautiful and powerful. But then they could start to admire them and learn some things about confidence from them. The Sailor Scouts, on the other hand, would be curious about the strange relationships in Ranma 1/2, especially the whole fiancée situation. They might try to help sort things out in their own way.
If we consider Akane, she could potentially have some interesting interactions with the Sailor Scouts too. She might envy their grace and power at first. But then she could learn from them and in turn, teach them about her own strength in martial arts. And for the villains in the Sailor Moon universe, they would see Ranma and his friends as either a threat or a potential tool to use in their evil plans. They might try to manipulate Ranma's curse or use his skills for their own gain.
Ranma is hot - headed and impulsive. He would probably clash with Sailor Moon's more emotional and sometimes naive personality at first. But over time, they could learn from each other. Ranma could teach her to be more assertive in a fight, while she could teach him to be more in touch with his feelings.
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.
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.
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.