When Lucy dies in a fanfiction, it might cause a rift among the guild members. Some may blame themselves for not being able to protect her. Her death could also be the start of a new adventure as the characters search for a way to resurrect her. This could lead them to explore new magical realms or encounter new enemies. Plus, it could change the power balance in the guild as her celestial magic was quite unique and important.
The guild as a whole would be in mourning. They would hold a big memorial for her. Her death could also inspire some of the younger members to train harder so that they can prevent such tragedies in the future. For instance, Wendy might focus more on her healing magic, hoping to be able to save lives in similar situations in the future.
It can completely change the mood. If Gajeel dies, it brings a sense of gloom to the usually upbeat Fairy Tail world. The guild members would be in mourning, and their actions would be centered around avenging his death or dealing with the loss. For example, Pantherlily might go on a quest for justice.
Happy, Natsu's companion, would also be very sad. He was used to Lucy being around and would miss her presence. The whole Fairy Tail guild would be in mourning. Erza, for instance, would feel a great sense of loss as Lucy was like a little sister to her. This could lead to changes in the guild's dynamics, with some members becoming more withdrawn while others might try to be stronger to honor Lucy's memory.
Some writers justify Lucy's death by making it a noble sacrifice. For example, she might give up her life to close a portal that would otherwise release a great evil upon the world. This way, her death is seen as necessary for the greater good of the entire Fairy Tail universe, including all the kingdoms and innocent people in it.
It can add new plot elements. For instance, Envy's presence might introduce new conflicts that the Fairy Tail characters have to face. This could change the direction of some story arcs in the original Fairy Tail.
It can also have a big impact on the characters. For those who have been directly affected by Acnologia's actions, like the dragon slayers who have a special connection to dragons and thus to Acnologia, his death can be a very emotional moment. They might struggle with mixed feelings of relief and guilt. Relief that the threat is over, but guilt because they may feel they could have done more or differently. This internal conflict can be explored in the fanfiction and add depth to their characters.
In many fanfictions, Natsu protects Lucy by using his powerful fire magic against their enemies. For example, if they encounter a dark guild attacking Lucy, Natsu would jump in front of her and unleash a flurry of fire attacks like his 'Fire Dragon's Roar' to drive the enemies away.
Lucy could save everyone by making a great sacrifice. Maybe she gives up her magic power temporarily or even permanently to stop an evil force. In fanfiction, this could be something really emotional. She might be the only one who can access a certain magical item or place because of her Celestial Spirit keys. By using this advantage, she gets to the root of the problem and saves the day.
In some fanfiction, Bickslow teases Lucy a lot, which is his way of showing he likes her presence. He might make fun of her in a light - hearted way about how she gets flustered around Natsu.
In some fanfictions, Lucy might use her Celestial Spirit magic. For example, she could summon a powerful spirit like Leo to help fight off whatever is threatening Natsu. The spirit could provide a distraction or directly combat the enemy while Lucy tries to reach Natsu and get him to safety.