One idea could be a crossover where Naruto and Lucy end up in the same world. Maybe they team up to fight a common enemy that threatens both their universes. They could learn from each other's unique abilities, like Naruto's jutsu and Lucy's Celestial Spirit Magic.
I'm not sure specifically as there could be many different interpretations in such a fanfiction. It might be a story where Naruto somehow becomes the father figure to Lucy, perhaps in a crossover universe where the Naruto and Fairy Tail (since Lucy is from Fairy Tail) worlds collide, and Naruto takes on a paternal role for some convoluted plot reasons.
A possible story could be that they planned for the pregnancy. They've been through so much together in their adventures that they feel ready to start a family. Their pregnancy journey could be filled with small, sweet moments like Wyatt talking to the baby bump at night, and Lucy writing letters to their future child about their time - traveling experiences.
One idea could be that Naruto and Lucy end up in a parallel dimension where they have to team up to defeat a common enemy. Naruto's powerful jutsu combined with Lucy's celestial spirit magic would make for some epic battle scenes.
One idea could be that Naruto and Lucy end up in a parallel universe where they have to combine their powers to find a way back home. Naruto's ninja skills and Lucy's celestial magic could be used in creative ways to overcome the obstacles in this strange new world.
A plot could involve Lucy getting into trouble with a powerful dark magic user. Yukino, being the loyal friend she is, comes to her rescue. They have to fight against the dark magic user and his minions. In the process, they learn more about each other's pasts and the reasons why they are so determined to protect the magical world. This experience deepens their bond and makes them an even stronger team in the magical community.
One idea could be that Lucy's little sister has a special magical ability that is different from Lucy's. For example, while Lucy uses celestial spirit magic, her sister could have nature - based magic. In the fanfiction, she might start out as a bit of an outcast in the guild because her powers are so different, but then she gradually earns everyone's respect through her determination and kind heart.
Perhaps Lucy time travels to a prehistoric era. She has to quickly adapt to survive among the dinosaurs. Along the way, she discovers that there are ancient secrets or powers hidden in that time that have an impact on her own time. She then has to figure out how to use this knowledge when she returns home.
Another plot idea is that Lucy, while on a mission, discovers a strange magical artifact that has a connection to Naruto's world. She accidentally activates it and it summons Naruto. Together, they have to figure out how to send Naruto back while also dealing with the chaos that the artifact's appearance has caused in the Fairy Tail guild. For example, it could be causing some of the guild members to have strange powers or behaviors similar to those in Naruto's world.
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.
Naruto would probably be very enthusiastic and friendly towards Lucy right away. He might be curious about her celestial spirit keys and ask a lot of questions. Lucy, on the other hand, might be a bit more cautious at first but would be drawn to Naruto's positive energy.