Her relationship with Natsu would likely change significantly. Since Natsu is a dragon slayer, they might become rivals in a friendly way, constantly trying to outdo each other in terms of dragon slaying skills. But they could also team up more often, combining their unique abilities to face greater threats.
One possibility is that Lucy discovers a hidden power within herself that is related to dragon slaying. Maybe she has a long - lost ancestor who was a dragon slayer. This newfound power could lead her on a journey to learn more about it, facing challenges along the way, like dealing with other mages who are jealous or scared of her new abilities.
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.
Lucy's new power would bring a shift in her relationships. With Happy, he might be constantly curious about her Celestial Dragon Slayer magic and pester her with questions. The guild as a whole would view her in a new light. Some might be jealous of her newfound power, while others, like Levy, would be excited to research more about it. This could lead to new friendships forming as Lucy shares her experiences with others in Fairy Tail, and also cause some minor frictions as she has to navigate through the different reactions to her power.
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.
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.
His relationship with Bakugo might change. Bakugo, who used to look down on Izuku, might now be a bit jealous or even more competitive as Izuku's Dragon Slayer quirk is very powerful. With his friends like Uraraka, she could be in awe of his new quirk and support him more actively in battles. Izuku might also attract the attention of some of the more powerful heroes in training, who want to team up with him because of his unique ability.
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.
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.