Well, it could start with Lucy being in a situation where she witnesses the destruction caused by a celestial dragon. This event stirs something deep within her, and she decides she wants to be able to protect everyone from such a threat. So, she embarks on a journey to become a celestial dragon slayer. She may travel to different realms or seek out rare magical items that can enhance her abilities. As she progresses, she could form alliances with other powerful mages who share her goal or have knowledge about celestial dragons. And gradually, she builds her skills and reputation as a celestial dragon slayer.
Perhaps an ancient sorcerer who has the ability to control celestial dragons. He sees Lucy as an obstacle to his domination over the world using the power of these dragons. He would use his vast knowledge of magic and his control over the dragons to try and defeat her.
Well, if we consider Lucy as a Dragon Slayer in fanfiction, her magic could be really different from the traditional ones. Maybe she'd have a connection to a celestial dragon, which would be so cool. It would combine her existing celestial spirit magic with the new dragon slayer powers. She could use the stars as a source of power, similar to how other dragon slayers use elements, and it would be a fresh take on her character.
Other dragon slayers could be her potential allies. They might recognize her skills despite her being a princess. Also, magical creatures who are enemies of the dragons, like phoenixes or griffins, could side with her. And some wise old wizards or witches who see her potential might offer their help.
Another theme could be friendship. In the context of being a dragon slayer, Lucy's relationships with her friends, whether they are also slayers or just her regular comrades, are likely to be explored. Her friends might support her in her new role, or there could be some conflicts related to her new power. Usually, how these relationships evolve is an important part of the fanfiction.
One possible scenario in the fanfiction could be that Lucy discovers she has latent dragon slayer abilities. This could start with strange dreams or unexplained powers she starts to exhibit. Her journey to understand and control these powers would be filled with self - discovery.
She could gain water dragon slayer powers after a magical accident. For instance, if she was caught in a magical explosion near a water - based artifact. Her powers could include being able to breathe underwater, which would open up new exploration opportunities for her and the guild. She could discover underwater ruins or hidden treasures.
Another possibility is that Lucy's Water Dragon Slayer powers are a result of a curse or a long - lost family heritage. As she delves deeper into understanding her powers, she uncovers secrets about her family's past. She might have to face off against an evil force that is also connected to her new powers. Along the way, she forms a special bond with the water element, seeing it as not just a tool for fighting but also a source of comfort and strength.
I'm not sure specifically which 'lucy is a dragon slayer' fanfiction is the best. However, some fanfictions that involve unique plot twists or in - depth character development tend to be really good. Maybe look for ones where Lucy has some extraordinary adventures as a dragon slayer and how it affects her relationships with other characters in the original series.