Maybe in some fanfictions, Future Lucy is killed by a new and extremely dangerous enemy. This enemy could be targeting her specifically because of her knowledge of future events. They manage to catch her off - guard and end her life before she can warn the others.
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.
Another scenario could be related to time - travel glitches. Since she is from the future, there could be some unforeseen consequences of her being in the present. Her body might start to deteriorate due to the instability of time, and eventually she succumbs to this, resulting in her death.
Maybe Lucy finds a long - lost spell book that belonged to a powerful Celestial Mage in the past. The spells in this book are the key to saving everyone. She spends time deciphering the ancient spells and finally manages to use one that can nullify the threat facing Fairy Tail. It could be a spell that can erase the existence of a powerful dark magic or something like that.
Another scenario could be that in a battle against a new, extremely powerful foe, Lucy is caught in a crossfire between two massive magical attacks. Despite her friends' attempts to protect her, she gets fatally injured and dies. This would have a huge impact on the other characters in the Fairy Tail guild, leading to themes of grief, revenge, and moving on.
One possible scenario could be Juvia sacrificing herself to save Gray. In the heat of a battle against a powerful enemy, Juvia might use her water magic to shield Gray from a fatal attack, but at the cost of her own life. Another could be that she succumbs to a dark magic that she was trying to fight off alone, not wanting to endanger her friends. Maybe she gets caught in a cross - fire between two extremely powerful magic users and her body is too weak to withstand the impact.
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.
Lucy could also cry when she faces betrayal. In fanfiction, there might be a new character who befriends her and then turns out to have ulterior motives. This sense of being deceived and used would be a big blow to her, and she would likely break down in tears. And sometimes, it's just the overall atmosphere of the story. If the fanfiction is set in a really dark and hopeless time in the Fairy Tail world, with the guild in danger and no clear way out, Lucy might cry out of fear and worry for her friends and the future of the guild.
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.
Another scenario might be a situation where Natsu accidentally eats some magic - enhancing food that makes his emotions go wild. Lucy has to figure out how to calm him down and in the process, they realize deeper feelings for each other. For example, Natsu might start uncontrollably shooting fire everywhere, and Lucy uses her whip to lead him to a safe place where she can try to reverse the effect.
One popular scenario could be during a dangerous mission. Maybe they're trapped in a magical cave, thinking they might not make it out alive. In that moment of high emotion and closeness, they share a kiss. It's a way to express their unspoken feelings for each other that have been building up throughout their adventures together.
The power balance in the fanfiction world could be different. Without Lucy, Fairy Tail might lack some of the strategic and magical capabilities she would have brought. At the same time, Lucy could be a major force in another part of the fictional world, changing the political and magical landscape there. It also means that the relationships and story arcs that typically involve Lucy in the original Fairy Tail story will be completely different, creating a whole new set of stories and interactions.