Maybe Lucy gets a special dragon slayer weapon that only she can wield. With this weapon, she teams up with Natsu and the others to fight a group of dragon-like creatures that are terrorizing the land. As she uses her new abilities, she also learns more about the history of dragon slayers.
One common theme is probably adventure. Lucy and the fire dragon slayer could go on quests together. Another might be friendship, as they develop a bond. And of course, there could be a touch of romance in some of the fanfics.
There are probably some out there. A lot of fanfic writers are inspired by the idea of Lucy and a fire dragon slayer. You might have to dig around a bit, though. Sometimes it depends on how popular the pairing or concept is within the fandom at the time. Some of the stories could be about Lucy teaming up with a fire dragon slayer on an adventure, or there could be a more romantic take on their relationship.
One interesting aspect could be how she trains to become a Celestial Dragon Slayer. Maybe she discovers an ancient magic that combines the two, and she has to learn to control the newfound power. This could lead to some great character development as she struggles with the increased power and the responsibility that comes with it.
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.
In some fanfictions, Happy, the flying cat, could also be an important character. He could provide comic relief and also be involved in some of Lucy's adventures as a dragon slayer. For example, he might help her find clues or get out of difficult situations. And then there could be original characters created specifically for the fanfiction. These characters could be other dragon slayers or powerful mages that interact with Lucy in different ways.
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.
There could be a 'Dragon Slayer Lucy' fanfiction where she embarks on a quest to find a missing dragon slayer. Along the way, she uncovers secrets about the origin of dragon slayers. She might also develop a romantic relationship with another character in the story, perhaps a fellow mage who helps her on her journey. This relationship could add an interesting layer to the story as they face various obstacles together.