One possible scenario could be that she sacrifices herself to save her friends from a powerful dark magic. In the chaos of a huge battle, she might use all her magic power to create a shield that protects everyone else but drains her life force completely.
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.
In many fanfictions, Future Lucy starts off as a determined and somewhat broken character. She has seen the destruction of her world and is desperate to change it. As the story progresses, she learns to trust the people in the past. For example, she might initially be hesitant to work with Natsu and the others, but as she sees their kindness and determination, she becomes more open. This helps her character grow from a lone wolf trying to save the world to someone who is part of a team.
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.
In some fanfictions of Lucy's death in Fairy Tail, a heartbreaking scene could be when her friends find out she sacrificed herself to save the guild. They might come across her lifeless body surrounded by the remnants of the great battle she fought. It's a moment of shock and extreme grief for them, realizing they've lost such an important member.
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.
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.
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.
Well, in some fanfictions, a plot twist could be that Future Lucy actually has a hidden power that she didn't know about until she traveled to the past. This power then becomes crucial in solving the problems in the past and also has implications for her own future. Also, there might be a plot twist where she discovers that the reason for her coming to the past was not what she initially thought. Maybe it was all part of a bigger plan by a character who has been manipulating events from the shadows all along.
One popular theme is Lucy's growth as a wizard. Fans often explore how she overcomes new challenges and becomes stronger. Another theme is her relationships, not just romantic ones but also her friendships within the guild. For example, her bond with Natsu and how they support each other through tough times.