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 plot could be that they decide to keep the pregnancy a secret at first. But as Lucy's body starts to show signs, it becomes harder to hide. They struggle with when and how to tell everyone, all while preparing for the arrival of their baby, like making a small nursery in a hidden part of their home.
Perhaps in the fanfiction, their pregnancy is a result of some magical accident. They then have to deal with the reactions of their friends and family. Some might be supportive while others are skeptical. Lucy and Rogue would have to prove that they are ready to take on this new chapter in their lives.
Lucy, on the other hand, might become more independent yet also more vulnerable. She is used to being strong on her own, but now she has to rely on Rogue and others more. This pregnancy could also strengthen her magical powers in some unexpected ways as her body is changing due to the pregnancy.
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.
A possible plotline could be that Lucy Chen decides to keep her pregnancy a secret at first. She's worried about how it will affect her work and her standing in the department. As she progresses through the pregnancy, it becomes harder to hide, and she has to face the consequences.
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.
One possible plotline could be that Lucy and Caspian are newly married and the pregnancy is a surprise. They then have to deal with the political implications in Narnia as well as their own personal fears and joys. Another could be that they face some sort of danger during the pregnancy, like an enemy threat that endangers Lucy and the unborn child.
One idea could be that Future Rogue travels back in time and meets Lucy before the events of the main story. He initially hides his true identity but gradually forms a bond with her as they go on adventures together. They might face a common enemy that only their combined powers can defeat.
One idea could be that Lucy and Rogue are brought together by a powerful magic artifact. They have to team up to protect it from an evil force that wants to use it for destruction. Along the way, they start to understand each other's pasts and form a deep bond.
One possible plotline could be that Lucy's pregnancy is a result of some magical interference in Narnia. Maybe a powerful spell was cast on her accidentally or as part of an evil plot. And she has to deal with the consequences while also trying to protect Narnia from an upcoming threat.
One idea could be that they discover the pregnancy unexpectedly. Maybe they were in the middle of a big adventure in their magical world and then Lucy starts showing signs. It throws them into a new set of challenges as they have to balance their duties in their guild with preparing for a new life.
One idea could be that Lucy discovers she's pregnant during a difficult time in the magical world. Gray is initially shocked but then becomes very protective. They might face challenges from their guild mates who are worried about how this will affect their ability to go on missions.