Perhaps the rogue is hired by an evil force to kidnap Lucy. She could be the daughter of an important figure or a person with unique abilities that the evil force wants to use against the good side. As the story progresses, Lucy could try to escape or even try to turn the rogue to her side by appealing to his better nature.
The rogue could be a disgruntled ex - knight who was wrongfully banished from his order. He sees kidnapping Lucy as a way to get revenge on the kingdom or the people who wronged him.
Well, it could be that Lucy has a misunderstanding. Maybe she overhears something that makes her think Natsu is planning to leave the guild without saying goodbye. So, in a fit of panic and anger, she kidnaps him to get an explanation. Then, as the story progresses, they have a heart - to - heart conversation where the misunderstanding is cleared up. It could also be a situation where Lucy is competing with someone else for Natsu's attention, and she thinks that by kidnapping him, she can have him all to herself for a while and figure out how to make him like her more.
One idea could be that Rogue discovers Lucy is pregnant and is initially in shock but then becomes extremely protective. He might start learning about how to take care of a pregnant woman and the coming baby. For example, he could read books on pregnancy and baby care in secret.
Another plotline might involve Karasu having a personal vendetta against Keiko or someone close to her. He kidnaps her to exact revenge in a more elaborate and drawn - out way. This could lead to a series of events where Keiko tries to outwit Karasu and escape while also uncovering the true reasons behind his actions. And during this process, there could be unexpected alliances and betrayals.
Another plotline might involve Tom believing that Harry has some hidden power that he can extract or use for his own dark purposes. So, he kidnaps Harry to figure out how to access that power. This could lead to a series of attempts by Tom to break Harry's will and get him to reveal the secret. For example, he might put Harry in a magically sealed room and use various spells to try and force the information out of him. Harry, on the other hand, could be trying to resist and find a way to escape or signal for help.
One possible plotline could be Buffy's friends rallying together to rescue her. They might use their unique skills, like Willow's magic, to track down Angelus and his lair.
One possible plotline could be Bella's friends and the Cullens coming together to rescue her. They might plan a stealthy infiltration of Aro's lair, using their unique abilities. For example, Alice could use her visions to anticipate Aro's moves and find the safest way in.
One possible plotline could be that Cato kidnaps Katniss as a strategic move in the Hunger Games. Maybe he thinks he can use her to draw out other tributes or get an advantage over the Careers. Another could be that Cato has some sort of personal vendetta against Katniss that leads him to kidnap her outside of the normal rules of the Games.
One possible plotline could be that the oc has a different set of skills or powers compared to Lucy, which changes the way the story unfolds. For example, if the oc is a magic user with a more aggressive style in a world where Lucy was more of a support - type magic user.
One possible plotline could be that Lucy has a hidden talent for singing, and in the Twilight world, she uses her singing to attract the attention of a certain vampire or werewolf. Maybe it creates a new kind of bond or rivalry among the supernatural beings.