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.
The plot could involve them discovering their brotherhood. Maybe they start off in different worlds and then some magical or mysterious event brings them together. They then realize they are brothers and start to explore their combined powers to fight against a common enemy.
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.
The two main characters are Naruto and Natsu. Naruto has his friends like Sasuke and Sakura in his world, but in this fanfiction, his connection with Natsu is what matters most. Natsu also has his Fairy Tail guild mates, but here, his newfound brother Naruto is the key relationship. Their interactions, their differences in powers and personalities, and how they support each other would be explored through their characters.
In this fanfiction, it could start with Naruto feeling a strange connection to Natsu. As they meet, they discover their shared blood relation. Then, they embark on a journey to find out more about their family's past. Along the way, they encounter various challenges like facing off against enemies who are after their unique powers. They rely on each other's strengths, Naruto's strategic thinking and Natsu's brute force and magic, to survive and grow closer as brothers.
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.
Natsu could inherit the ability to use chakra. This would give him an extra edge in combat, allowing him to perform jutsus like Naruto. Maybe he could even learn the Rasengan, which he could combine with his fire magic for a super - powerful attack.
Well, it's quite possible. Fanfiction writers are known for their wild and creative ideas. Since both Naruto and Natsu are from popular anime, some writer might have thought of creating a story where Naruto is somehow related as Natsu's grandfather. But it also depends on how much of the respective anime universes the writer wants to blend and how they would explain such a connection in the story.
Yes, there are likely fanfictions out there with such a concept. Fanfiction platforms are full of creative and often wild ideas. You can check major fanfiction websites like FanFiction.net or Archive of Our Own (AO3). Just search for the relevant tags or keywords to find them.
Fanfiction often takes characters and gives them new relationships. Regarding Naruto being Anko's brother, it depends on the specific fanfiction. Some authors might see an interesting plot point in making them siblings. For example, they could write a story where Naruto's positive attitude affects Anko in a new way because of their brother - sister bond. Or perhaps they explore how their shared heritage (if fabricated in the fanfic) influences their ninja paths. But it's important to note that this is not the case in the original Naruto anime and manga.