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.
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.
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.
Yes, there are likely some out there. You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. Just search for the relevant tags like 'Naruto', 'Fairy Tail', and 'harem' to find such fanfictions.
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.
It could be Zeref from Fairy Tail. His dark magic combined with his immortality would make him a formidable foe for a godlike Naruto. He might see Naruto's power as a threat and try to eliminate him, or use him to achieve his own dark goals.
Perhaps it could be a character who is a hybrid of Naruto and Ichigo. He has Naruto's determination and Ichigo's cool - headedness in battle. His unique heritage gives him the ability to use both ninjutsu and zanpakuto skills. This makes him stand out and draw the attention of female characters from both series, making him the main character in this harem - filled fanfiction.
Obviously, Naruto is one of the main characters as the title mentions him leaving. And in the context of Fairy Tail, characters like Natsu, Lucy, and Erza are likely to be important too. They would probably interact with Naruto during his time in Fairy Tail and might be part of the reason he decides to leave, either through their relationships or the events they are involved in together.
Perhaps the story could involve a powerful entity from the Naruto universe arriving in the Fairy Tail world. This entity, with godlike powers, is on a quest. Along the way, he meets different Fairy Tail characters. As he helps them with their struggles against dark guilds or powerful enemies, he earns the affection of multiple female characters, creating a harem situation. The story could then explore how he manages these relationships while still staying true to his own goals.
One possible plot could be that Naruto, with his godlike powers, ends up in the Fairy Tail world by accident. He's lazy at first, not really wanting to get involved in their adventures. But as time goes on, he sees the importance of using his powers to help his new friends in Fairy Tail against a powerful enemy. Maybe he starts training some of the Fairy Tail members with his unique techniques too.
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.