Well, a really cool crossover might be Gray with a Zoan - type Devil Fruit that turns him into an ice - related creature. It could be something like an ice - dragon - like form which would blend well with his ice - make magic. Also, Erza could have a Devil Fruit that gives her the power to change her armors instantly just by thinking, a kind of Paramecia power related to her equipment. It would make her battles even more efficient and unpredictable.
For characters like Wendy, if she got a Devil Fruit power, she'd need to be careful not to overexert herself. Since she's a healer, she might get a Paramecia - type Devil Fruit that gives her the ability to heal more effectively. But she'd have to learn how to balance it with her Sky Dragon Slayer Magic. And characters like Elfman would need to find a way to use their Devil Fruit power in harmony with their Take - Over Magic. If he got a Zoan - type Devil Fruit, he'd need to figure out how to transform between his normal form, his Take - Over forms, and his Devil Fruit form without getting confused or losing control.
Well, without reading the actual fanfiction, it's hard to say exactly. But generally, it could be that the author wanted to combine the popular Naruto character with the magical and often wild world of Fairy Tail and added a devilish twist to it. Maybe Naruto is a devil in this fanfic, which would change his personality and abilities completely, and the story could be about how he navigates the Fairy Tail world with his new identity.
Maybe Naruto, as a devil, comes to the Fairy Tail world to cause chaos. He could be seeking power or revenge against something in that world. He might use his ninja skills in a twisted way, against the mages of Fairy Tail.
One common theme is the struggle between good and evil. Since devil slayers are involved, there's always the fight against dark forces. Another theme is friendship, like in Fairy Tail, where the devil slayers form bonds with the guild members.
Another common theme is adventure. Combining the worlds of Naruto and Fairy Tail creates a vast new universe full of unexplored places and new threats. So, many fanfictions send the characters on grand adventures in this new world. It usually involves them facing powerful enemies and finding new allies along the way.
One interesting character could be a Devil Slayer who has the power to manipulate shadows. He can use the darkness not only to attack demons but also to move stealthily. His backstory of how he got his powers, perhaps from a fateful encounter with a shadow - based demon, makes him really engaging.
Well, there are several places to search for this fanfiction. Firstly, as I mentioned before, FanFiction.net is a great place. It has a huge database of fanfictions from various fandoms. Another option is Archive of Our Own (AO3). It's known for its wide range of fan - made content. You could also try some Naruto or Fairy Tail - specific fan forums. Sometimes, fans share their own or other great fanfictions there. However, keep in mind that not all fanfictions are of high quality, so you might have to do some sifting through the results.
A common one is a Devil Slayer's journey to redeem themselves. Maybe they were once on the wrong side or made mistakes in the past. Joining Fairy Tail gives them a chance at redemption. They have to prove themselves worthy to the guild members while also using their Devil - Slaying skills to protect the guild from threats that target their new - found family.
It could be considered unique. Naruto and Fairy Tail are two distinct anime with their own unique worlds and characters. Combining Naruto as a 'devil' in a Fairy Tail fanfiction is not a typical or mainstream idea, which makes it stand out in the vast sea of fanfictions.