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.
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 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.
Dragon Slayer by Fire was a legendary mobile game that was based on the classic gameplay of Legend of Blood and Legend of Ice. It removed some unnecessary settings and quests, retaining classic gameplay such as treasure hunting, team battles, and sand attacks, allowing players to focus more on the core experience of the game. At the start of the game, only the Warrior class could be chosen. There were no choices for the Mage and Taoist classes. In this version with a single class at full attack speed, players could experience the feeling of five slashes per second. The Legend of the Fire and Dragon Slayer was an original single-class retro legendary mobile game. Battle pet summoning, field PK, full-service ladder, guild hegemony, and other gameplays all appeared, satisfying all the needs of the fragment time. There was a unique team system in the game, so players could fight the BOSS with their brothers and experience the high drop rate and the thrill of harvesting. In short, the Legend of the Fire Dragon Slayer was a legendary mobile game that could not be stopped. It allowed players to return to their passionate youth and use unlimited PK to defend their glory.
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.