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.
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.
It could work by making Naruto a more mysterious and brooding character. In Fairy Tail, there are many characters with different personalities. An emotionless Naruto would stand out. He might be more focused on achieving his goals without being distracted by emotions. For example, he could be on a mission to find a way back to his own world and his lack of emotion helps him stay determined.
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.
One possible plot could be that Naruto is transported to the Fairy Tail world and is somehow stripped of his emotions during the process. The Fairy Tail wizards then have to figure out how to send him back while also trying to restore his emotions. Along the way, Naruto's lack of emotion might cause him to make some uncharacteristic decisions that put everyone in danger.
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.