There could be a fellow time traveler as her love interest. They both have the same ability and they meet during different time - traveling adventures. They understand each other's experiences like no one else and their shared passion for exploring different times brings them closer together.
Natsu is probably the most common. Their chemistry in the original series makes it natural for fans to pair them in fanfictions. They have a lot of shared adventures and their personalities complement each other well.
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.
Lucy may also come across ancient warriors. For example, she could meet a Spartan soldier and be amazed by his combat skills and strict discipline. They could have a brief encounter where the Spartan is curious about this strange girl from another time.
Lucy could be an ordinary girl who stumbles upon the dragon by accident. Maybe she was just exploring an old forest or a mysterious cave when she came across it.
There's not enough information to really know who Lucy specifically is. She could be just a regular reader of fanfiction who took the time to rate this piece as M. Maybe she's part of a fanfiction community where people rate works to help others know what to expect.
Natsu could be a great ally. Since he is also a dragon slayer, he could offer advice and support to Lucy in her elemental dragon - slaying journey. Happy, Natsu's flying cat companion, could also be helpful in terms of providing transportation and some comic relief. Erza is another potential ally. Her strong combat skills and leadership could be valuable when Lucy faces tough dragon - slaying challenges.
Perhaps Lucy time travels to a prehistoric era. She has to quickly adapt to survive among the dinosaurs. Along the way, she discovers that there are ancient secrets or powers hidden in that time that have an impact on her own time. She then has to figure out how to use this knowledge when she returns home.
Perhaps a mortal with a special gift for metallurgy or some other craft related to Hephaestus' work. This mortal could be seen as an equal in terms of skill, at least in a way that catches Hephaestus' eye. They would bond over their shared love for the craft, and over time, this bond could turn into a deep, loving relationship. Hephaestus might be drawn to the mortal's unique perspective and the way they approach the craft despite not having the same divine powers as he does.
Some popular themes in Lucy fanfiction might include adventure. For example, Lucy could go on a magical journey in a fantasy world filled with strange creatures and hidden treasures. Another theme could be friendship, where Lucy forms deep bonds with other characters, and they support each other through various challenges. Romance can also be a common theme, perhaps with a mysterious character falling in love with Lucy and having to overcome obstacles to be together.
Natsu would be a very likely character. Since he and Lucy are close friends in the original story, if Lucy travels back in time, Natsu might be one of the first people she seeks out. They could have adventures together in the past, with Natsu being his usual impulsive self and Lucy having to keep him in check with her knowledge of the future.