It could be a heroic character she meets during her quest. Maybe a powerful wizard or a brave knight who has a brief but intense encounter with Lucy, leading to her pregnancy.
The fanfic might focus on the relationships around Lucy during her pregnancy in this 100 - year journey. Her friends or family could be helping her, or there could be some who oppose her due to the special circumstances of her pregnancy. For example, there could be a prophecy related to her unborn child that some want to prevent from coming true. Also, the passage of time during this 100 - year quest could have a profound impact on her pregnancy. She may experience different stages of the pregnancy in different historical or magical periods, which could bring about unique difficulties and growth for her character.
A Natsu x Lucy pregnant fanfic could be full of heartwarming moments. Think about Natsu being overprotective of Lucy during her pregnancy. He'd probably be a bit clumsy at first, but his love for her would shine through. And Lucy would be the one trying to keep things in order while also dealing with her hormonal changes. It could also add a new dynamic to the Fairy Tail group as a whole.
The father of the baby could be a major character. He could be a loving partner, or there could be some drama if he's not as committed as Lucy. Her best friend could also be in it. The best friend might be the one she confides in about all her pregnancy fears and joys.
The baby's father, whoever he is, could be another main character. He might be a mysterious figure at first, and as the story progresses, his identity and his relationship with Lucy get explored more deeply. He could be a guy from Lucy's past that she has complicated feelings for.
Well, one possible plot could be about Lucy's journey through her pregnancy. Maybe it focuses on how she prepares for the baby, like decorating the nursery. It could also show her dealing with the physical and emotional changes. For example, she might be excited but also a bit scared at the same time.
I'm not sure if there is a specific fanfic with that exact plot. There are a vast number of fanfics out there in the Fairy Tail fandom, but it would take some searching through various fanfic platforms to know for sure. Some popular fanfic sites like Archive of Our Own or FanFiction.net might be good places to start looking if you're interested in such a story.
One plot idea could be that a magic spell goes wrong and Lucy ends up pregnant. Natsu is initially shocked but then quickly embraces the idea of being a father. They then have to deal with the guild's reactions, some of which are positive and some negative.
It's highly likely that there exists a fanfic with a Nalu and Lucy pregnancy plot. Given the large number of fanfic writers and the popularity of the Nalu ship, many fans like to create alternate storylines for the characters. Some might be interested in exploring the idea of Lucy becoming pregnant, perhaps dealing with how Natsu and Lucy would handle the situation, their reactions, and how it would affect their relationship and the world of Fairy Tail around them.
A theme could also be the reaction of their friends and the community in the Fairy Tail world. Their friends might be excited, offer help, or there could be some drama if not everyone is supportive. This would add an extra layer of story - telling as they navigate not only their own feelings but also the opinions and actions of those around them.