It could be Lucy. Since she's a central character in Fairy Tail, she could be the one who gets involved with the mystery of the red nails. Maybe she finds a pair of red - nailed gloves at a magic market and they start to give her strange powers or visions.
Erza could be the main character too. Given her strong and fearless nature, she could be the one who is tasked with protecting someone with red nails. It could be that a new member joins Fairy Tail with red nails that have a special significance, and Erza takes it upon herself to keep this person safe from those who would use the power of the red nails for evil.
One idea could be a story where the red nails have a magical origin. Maybe they were once part of a powerful artifact and the main character in the Fairy Tail universe discovers their true power and has to protect them from an evil force that wants to use them for destruction.
Gray could be the main character in a harem fanfiction. His cool exterior and his ability to use ice magic in such a unique way make him stand out. He has a bit of a rivalry with Natsu which could add an interesting dynamic to the harem. Characters might be attracted to his brooding nature and his underlying loyalty to his friends and the guild.
It could be a new character, like a street - smart orphan who has learned to steal to survive. This character sees Fairy Tail as a source of wealth at first but then discovers there's more to the guild than just gold and jewels. Another option is an ex - member of a rival guild who turns to thievery after being exiled and decides to target Fairy Tail for revenge. Or it could be a magical creature, like a mischievous imp with the power of invisibility that steals from Fairy Tail just for fun.
Lucy would also be a great main character. Since she has a collection of celestial spirits, adding a genie to her story would be fascinating. She could struggle with whether to use the genie's wishes or rely on her spirits, and the genie might be jealous of her celestial spirits, leading to conflict and character development.
Natsu could be an interesting choice. Imagine if he accidentally caused a great deal of damage during a battle and had to make up for it. He would use his fire - dragon - slayer magic in new ways to help the guild and the people they protect.
Natsu could be a great main character for a Fairy Tail wolf fanfiction. Picture Natsu befriending a wild wolf in the forest that has some kind of fire - related power. Natsu and the wolf could team up to fight against stronger enemies. They might learn from each other and develop new fighting techniques. Erza could also be a main character. She could find a magical wolf armor that gives her enhanced strength and speed, and she has to master using it while also protecting the wolf that is somehow connected to the armor. Additionally, a mysterious outsider who has the power to control wolves could be the main character, bringing a new and dangerous element to the Fairy Tail world.
Lucy could be a great main character. Since she has a collection of celestial keys, the Rinnegan could give her new ways to interact with her spirits. For example, she might be able to use the Rinnegan to enter the spirit realm directly without using the keys, or enhance the power of her spirits when they are summoned. It would be an interesting twist on her character and her abilities.
It could be an original angel character that you create. This angel could have special powers like the ability to heal any wound or the power to manipulate light in unique ways. They would enter the Fairy Tail world and become the central figure in the story, interacting with all the well - known Fairy Tail characters.
Natsu could be a great main character. He's always hungry and loves food. He could be on a quest to find the ultimate cheeseburger in the Fairy Tail world. Maybe it's hidden in a dangerous dungeon, and he has to battle through all kinds of monsters just to get a taste of it.
Natsu could be a great main character. Since he already has a strong connection to fire, his encounter with the Flame God could lead to him unlocking new levels of power. He could learn unique fire - based spells and techniques from the Flame God, which would be exciting to read about.