Well, it would be an interesting concept. Lucy is a very popular character in Fairy Tail. If someone was reborn as her in a fanfiction, they might have to deal with her magic, her relationships with the other guild members like Natsu and Erza. They could potentially change the course of the story, for example, by using modern knowledge to solve problems in the Fairy Tail world more efficiently.
It would be amazing. With a system, you could gain unique powers and abilities that are not available in the normal Fairy Tail world. For example, you might be able to level up your magic faster or have special skills like instant teleportation within the guild. You could also use the system to analyze your opponents' weaknesses, making you a formidable mage in battles.
One possibility could be that a main character, perhaps Natsu, is somehow reborn without emotions. He would then go through the world of Fairy Tail in a very different way. For example, he might not be as eager to fight for his friends because he doesn't feel the passion or loyalty he once did. This could lead to a lot of conflict within the guild as they try to figure out how to bring back the old Natsu.
On the other hand, there's a possibility of them having a change of heart. The positive and friendly environment of Fairy Tail could gradually influence them. They might start to form real bonds with the characters and slowly let go of their psychopathic tendencies. The Fairy Tail members' acceptance and kindness could be the key to their redemption.
It could be an ancient evil entity that has been sealed away for centuries. The OC's rebirth somehow disturbs the seal, and this entity awakens. It views the Fairy Tail guild as a threat because of their positive influence in the world. So, it starts to target the guild, and especially the OC, as they are a new and unknown factor in the fight against it.
Well, perhaps the OC is reborn into Fairy Tail as a child who has the power to communicate with spirits in a different way than Lucy. This new ability could attract the attention of the guild. As they grow up in the guild, they learn about friendship and loyalty. They might face challenges from other dark guilds who want to use their special power for evil. Through the help of their Fairy Tail friends, they overcome these difficulties and become a key member of the guild.
Natsu could be a great choice. He's a central figure in Fairy Tail. His usually fiery and passionate personality being replaced with an emotionless state would create a lot of contrast and interesting storylines. The guild members would be shocked to see him so different, and his lack of emotion could also affect his relationship with Happy.
It adds more content. The official Fairy Tail series has a limited number of episodes and chapters. Fanfiction fills in the gaps. For example, it can create stories about what the characters were doing during the time - jumps in the original story.
If Lucy never joined Fairy Tail, the friendships and relationships among the characters would form in a very different way. Also, many of the story arcs might not unfold as they do in the original plot.
One popular theme is alternate universes (AUs). Writers like to explore what would happen if the Fairy Tail characters were in different settings, like a modern - day high school or a steampunk world. Another common theme is romance. Fans often pair up their favorite characters in different relationships, such as Natsu and Lucy or Gray and Juvia.