It could work if there was a shared backstory created for them. Perhaps they both have a common ancestor or some ancient power that links them. Naruto's strong sense of loyalty and protection could be extended to Natsu as his younger brother. He could teach Natsu some of his ninja values like never giving up and the importance of teamwork. Natsu, in turn, could show Naruto the more carefree and adventurous side of the Fairy Tail world.
If Naruto were the older brother of Natsu, their personalities would play a big role. Naruto is often boisterous and full of energy, which is similar to Natsu. However, Naruto has a more strategic mind from his ninja training. He could use this to help Natsu out of tricky situations. For instance, when Natsu is about to rush headlong into a battle, Naruto could use his wisdom to analyze the situation first and come up with a better plan. This dynamic could create a really engaging fanfiction.
Maybe they embark on a journey to save their respective worlds from a common threat. A powerful demon or an evil wizard is trying to destroy both the Naruto universe and the Fairy Tail world. Naruto and Natsu would travel across different dimensions, gathering allies from both worlds. Naruto would use his Rasengan and Natsu his Fire Dragon's Roar to fight off hordes of minions while they search for a way to stop the main villain.
Well, one way it could work is if there was some sort of multiverse crossover concept. Maybe in a world where different fictional universes collide. Naruto's skills like his shadow clone jutsu could be used in interesting ways to help Wonder Woman in battles. He could also teach her about the importance of friendship and determination which are big themes in his story.
Well, in this fanfiction concept, it could work in terms of Naruto's protective nature. Naruto is known for his loyalty and his will to protect those he cares about. If he were Erza's older brother, he would surely use his skills and abilities to keep her safe. He could teach her some of his ninja techniques, which would be really cool to see combined with Erza's own combat skills. Also, Naruto's positive attitude could influence Erza in a different way, making her more open and perhaps less brooding at times.
The older brother in the fanfiction might be a character with a tragic past. For example, he could have been experimented on by Orochimaru and gained some strange powers as a result. And he returns to the village later to protect Naruto from the threats that are coming towards him.
Yes, there are likely some fanfictions out there with this concept. The 'Naruto' and 'Fairy Tail' (where Natsu is from) fandoms are both quite large, and fans often like to create crossovers that connect different universes in interesting ways.
One idea could be that Naruto, with his strong sense of justice and loyalty, helps Rias in the supernatural politics of her world. He might use his ninja skills to protect her and her peerage from other devils or threats. For example, when facing a powerful rival devil clan, Naruto's shadow clone jutsu could create a diversion while Rias plans a strategic attack.
Yes, there might be such fanfictions out there. The idea of crossing over two very different anime universes like Naruto and High School DxD (where Issei is from) and creating a family - like bond between the characters is an interesting concept that fanfiction writers might explore.
Perhaps in this fanfiction, Naruto has to keep his identity a secret from Minato. He could be from an alternate future where things went wrong, and he's there to make sure Minato becomes Hokage and prevents certain disasters. So he has to subtly guide Minato without revealing who he really is.
In a fanfiction where Naruto is Madara's older brother, it could completely change the Naruto universe's dynamics. For example, Naruto's ideals of peace and friendship might have influenced Madara at a young age, preventing him from going down the dark path he did in the original story. Naruto could have taught Madara about the power of bonds, which was something Madara seemed to have lost sight of later in his life. This would lead to a different series of events, perhaps even averting the many wars that took place in the Naruto world.
If Naruto was the older brother of Kushina, it would rewrite a lot of the backstory. For one, Naruto might be the one to introduce Kushina to Minato. He could see Minato's potential and think he would be a good match for his sister. Also, Naruto's skills and abilities could have a different origin. Maybe he learned some things from Kushina or their family that were unique. This would change his fight scenes and how he approaches challenges in the fanfiction. He might also have a different relationship with the Uzumaki clan as a whole, having more of a family - based influence within it.