It could be that she had some sort of disagreement with the fanfiction community related to Fairy Tail. For example, there might have been conflicts about the direction of the storylines or character interpretations. This could have made her decide to quit.
Perhaps real - life obligations got in the way. She could have become too busy with school, work, or personal matters to continue writing or being involved in the Fairy Tail fanfiction. Fanfiction often requires a significant amount of time and dedication, and if she couldn't spare that time anymore, she would quit.
Maybe she just lost interest in the Fairy Tail world. Fanfiction can be a passing fancy, and her enthusiasm might have waned over time.
In a fanfiction, there could be a situation where Lucy is forced to side with a group that Natsu is against. This would make Natsu see her as an enemy. Or perhaps there is a prophecy that Lucy is involved in that Natsu misinterprets as something bad. He might think she is a threat to the guild or his friends. Also, if there is a love triangle situation where Lucy chooses someone else over Natsu in a way that really hurts his feelings, that could be a reason for him to start hating her.
It might make some readers who were fans of her work sad. They would no longer have her unique perspective on the Fairy Tail universe to look forward to.
One possible reason could be a misunderstanding. Maybe someone framed Lucy for something she didn't do, like stealing a powerful magic item from the guild. Another reason might be that she accidentally broke a very important and unbreakable guild rule without realizing it. For example, if there was a rule about not using a certain type of dark magic, and she unknowingly used it to save a friend.
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.
One possible reason could be that she feels she has outgrown the guild. Maybe she wants to explore new magic on her own and believes the Fairy Tail environment has become too comfortable and is hindering her growth.
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.
Well, one possibility could be that the story focuses on how Lucy adjusts to her new state. Maybe she has to cut back on her guild missions at first. She might worry about how Natsu and the others will react. It could also involve her having strange pregnancy cravings that lead to some comical moments in the guild. And as her pregnancy progresses, she could start preparing a nursery in her home, all while still being part of the Fairy Tail family.
In some fanfictions, Lucy's true power might be related to her celestial spirit heritage in a deeper way. Maybe she can merge with her spirits on a more profound level, not just summoning them for combat. This could give her enhanced physical and magical abilities.
Perhaps it's a power related to the stars in a more direct way. Like she can draw energy from specific constellations and use it to enhance her attacks. For example, if she summons the power of Orion, she gets super strength and accuracy in her attacks.
If Lucy never joined Fairy Tail, the friendships and relationships among the characters would form in a very different way. Also, many of the story arcs might not unfold as they do in the original plot.