It could also be a new character entirely. This character could be a hybrid of the two worlds, having some ninja abilities and some magic powers. Being a unique being, they would attract the attention of female characters from both Fairy Tail and Naruto. This new character's journey of self - discovery and their attempts to bridge the two worlds could be the central plot, with the harem aspect adding a romantic sub - plot.
It could be Naruto himself. Imagine Naruto, after achieving some sort of ultimate power in his own world, is transported to the Fairy Tail world. His energetic and kind - hearted nature would attract the female characters in Fairy Tail, like Lucy, Erza, etc. His ninja skills would be a novelty in that world and his godlike powers would make him stand out, leading to the formation of a harem.
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.
Erza from Fairy Tail could also be a main character in this type of fanfiction. She is strong, independent, and has a complex personality. In a Naruto x Fairy Tail crossover, she could form a harem of her own, with Naruto characters being attracted to her strength and Fairy Tail characters having their own relationships with her. Her leadership skills and magical abilities would play an important role in any story that unfolds.
The main characters could also be Sasuke from Naruto and Gray from Fairy Tail. Sasuke's cool and brooding nature combined with his Sharingan abilities would be a draw. Gray's ice - make magic and his habit of accidentally stripping could add some humor. In the harem, characters like Ino from Naruto and Levy from Fairy Tail could be part of it. Ino's mind - control jutsu and Levy's intelligence with books and runes would bring different elements to the story.
It could be a new character created specifically for this fanfiction. This character could be a young water dragon slayer who gets lost between the two worlds. They would be the main focus as they try to adapt to the Naruto world while still having their Fairy Tail powers.
It could be Sasuke. Since he has the Sharingan which is related to the Kamui ability in a way. He could be transported to the Fairy Tail world and his brooding and powerful nature would make for an interesting dynamic as he tries to find his way back home while getting involved in Fairy Tail's affairs.
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.