In the fanfiction, the person waking up as Lucy would have to face the challenges that Lucy herself has. This includes dealing with her past, like her relationship with her father. She would also have to manage her relationships with other guild members, such as Natsu and Erza. And of course, she'd need to keep up with her training to become stronger in magic. Maybe she'd even try to create new spells or improve on Lucy's existing ones.
One possibility is that they could change the course of the story. For example, if they know the future events from being a fan, they could prevent bad things from happening. They might also be able to form closer relationships with the other characters than Lucy did in the original story.
In a fanfiction where Lucy is kidnapped, the Fairy Tail guild would likely spring into action. Natsu, being her closest friend, would be furious and determined to find her. He'd probably charge off without much of a plan, with Happy flying after him trying to keep him in check. Gray would also be involved, using his ice - magic to combat any obstacles in their way. Erza would lead the strategic part of the rescue mission, planning their every move carefully.
In some fanfictions, if Natsu kidnapped Lucy, it could be a comical situation. Natsu might do it out of his own strange sense of protecting Lucy in an overzealous way. Maybe he thinks there's a danger that only he can keep her safe from, and his impulsive nature leads him to this 'kidnapping' which is more like a forced protection.
Perhaps the new keys are part of a prophecy. In the Fairy Tail universe, prophecies are common. So, Lucy finding these keys could be the start of fulfilling a great prophecy. It could involve saving the world from a dark magic that only she, with her new celestial spirits summoned by the keys, can stop. This would not only test her strength but also her loyalty to Fairy Tail and her friends.
Well, one possibility is that Ash wakes up with all the powers of Mew. He could then use these powers to quickly become the best Pokémon trainer ever. He might be able to communicate telepathically with all Pokémon, understand their needs and desires instantly, and use his newfound psychic abilities to battle more effectively. For example, he could levitate his Pokémon during battles, giving them an unexpected advantage.
Well, one possibility is that the fan might use Lucy's body to experience the magical world of Fairy Tail in a more immersive way. They could interact with the other characters, like Natsu and Erza, in a completely different dynamic since they have the body of a main character.
They might find a world full of advanced technology that they have a hard time understanding at first. For example, they could encounter flying cars or holographic communication devices.
She might try to prevent all the tragedies that befell her family. For example, she could warn Ned Stark about the Lannisters' schemes, which might change the whole course of events in Westeros. Maybe she would also use her knowledge of future alliances and enmities to her advantage, forging new relationships and breaking old, harmful ones.
Midnight might use his illusions to make Lucy think she's lost her Celestial Keys. This would be a huge psychological blow to her. But then Lucy could remember the bond she has with her spirits and break through the illusions. It would be a battle of wills as much as magic.
One possibility is that the guild would experience a significant emotional impact. Lucy is a central figure, and her celestial spirit magic has been crucial in many battles. Juvia, with her water magic, is also a strong member. Their departure might lead to a decrease in the guild's overall power. Some members might feel betrayed, while others could be motivated to find out why they left and try to bring them back.