It might be a power of illusion. Lucy could create illusions of herself or her celestial spirits to confuse her enemies. This would be a great addition to her combat skills as it would give her an element of surprise and misdirection.
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.
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.
If Lucy gets a new power, it will likely change the power dynamics in the Fairy Tail guild. She might be able to take on tougher opponents on her own, which could lead to more solo missions for her.
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.
Often, her true power is shown as an amplification of her celestial spirit magic. For instance, she might be able to call upon multiple spirits simultaneously without tiring, which is a significant upgrade from her normal abilities in the original series.
Maybe she just lost interest in the Fairy Tail world. Fanfiction can be a passing fancy, and her enthusiasm might have waned over time.
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, if we're following the common ships in the Fairy Tail fandom, Natsu could be a likely candidate. Their relationship is very close and there are many hints of a deeper connection between them in the original series. It would make for an interesting story as Natsu would have to grow up a bit to take on the role of a father.
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 possible plot could be that Lucy and Juvia leave Fairy Tail to go on a self - discovery journey. They might want to explore their individual magic powers more deeply without the constraints of the guild. Maybe they travel to different magical realms, meet new characters, and learn unique magic techniques that they can bring back to Fairy Tail later.
Well, in a fanfiction, Lucy might join Fairy Tail as a kid full of wide - eyed wonder. Maybe she stumbles upon the guild while on an adventure with her family. She could be immediately drawn to the warm and rowdy atmosphere of Fairy Tail. The members might see her potential and welcome her with open arms. She could start learning simple magic tricks from the more experienced wizards there.