One reason could be that the fanfic focuses on a different pairing for Lucy. So, to make that other relationship more plausible, the author made Lucy not have feelings for Natsu.
It might be due to a conflict in their values in the fanfic. If Natsu has values that are completely opposite to what Lucy stands for in that particular story, it could lead to her not loving him. For instance, if Natsu is overly reckless and it endangers Lucy or others she cares about, that could be a turn - off.
In the fanfic, there could be a new character who comes between them. This new character might have qualities that draw Lucy's attention away from Natsu. Maybe this new character is more in tune with Lucy's dreams and goals, while Natsu seems to be on a different path in the story.
One possible reason could be that Lucy is too focused on her own goals, like becoming a stronger mage or achieving something specific in the guild. She might see a romantic relationship as a distraction at that moment.
One reason could be for the shock value. Authors might want to create a very different and unexpected scenario to draw readers in. Another could be a misinterpretation of Natsu's passionate and brash nature as something more negative and controlling.
Perhaps Natsu is under the influence of a powerful and dark magic that makes him lose control of himself and accidentally hurt Lucy. It could also be that Natsu is trying to protect Lucy from something but his actions end up causing her harm instead. Like if he tries to keep her away from a dangerous situation by pushing her too hard and she gets injured.
One possible reason could be to seek new adventures. They might feel they've reached a point in Fairy Tail where they've done all they can and want to explore new magic and challenges elsewhere. For example, they could have heard of a far - off land with a different type of magic that they are eager to learn about.
A possible plot could be that their married life is disrupted by a mysterious force that is causing chaos in the magical world. Natsu and Lucy, being the strong couple they are, set out to find the source of this problem. As they journey, they discover secrets about their own powers and how their union might be the key to saving the world. There could be moments of doubt and fear, but their love for each other always pulls them through.
A possible plotline in this kind of fanfic could be quite complex. Firstly, there would be the moment of realization for Natsu and Lucy. After that, they would need to consider the practical aspects like where they will live and how they will support the baby. There could be a rift in their friendship group, with some friends being supportive and others being against the idea. As the pregnancy progresses, they might have to deal with health issues related to Lucy or the baby, and all the while, their relationship would be evolving in the face of this major life change.
Another reason could be that Hinata herself undergoes a significant transformation. Maybe she has a spiritual awakening or a change in her life goals that make her realize that her love for Naruto was more of an infatuation based on her younger self's ideals, and as she grows, she no longer feels the same way.
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.
In some fanfics, they might be considered a team within the guild. They are the ones often chosen for special and difficult missions. Their relationship is built on mutual respect for their individual abilities. Natsu's raw power, Erza's combat expertise, and Lucy's unique summoning magic make them a formidable trio that other guild members look up to.
Well, there are several potential plots for this kind of fanfiction. Maybe Natsu is in an environment where he is constantly bullied and abused, like in a guild where some members are jealous of his power. Lucy, who has a kind heart, decides to take him under her wing. She might teach him self - defense or help him build his self - esteem. Another plot could involve Natsu being abused due to a family secret or a prophecy that makes others target him. Lucy, with her intelligence and courage, tries to rewrite the fate that seems to doom Natsu to abuse. This could involve going on quests to find artifacts or information that can change the situation.