It could be another male character who is also interested in the same person as Natsu. This would create a love triangle. He might be from a different guild or have a different type of magic, and the competition between him and Natsu for the girl's love would be a major part of the story.
In some fanfictions, Zeref is portrayed as a more sympathetic character. Instead of just being the big bad, he might be shown as a victim of his own power or circumstances. For example, he could be a character who was forced into doing evil things and actually has a good heart deep down.
One possibility is that Naruto, with his determination, could adapt Zeref's powers in a unique way. Maybe he'd use Zeref - like magic to enhance his ninjutsu. For example, he could infuse Zeref's death - based magic into his Rasengan, creating a more powerful and dangerous attack. Another aspect could be Naruto's personality change. If he becomes influenced by Zeref, he might become more brooding and less of the cheerful ninja we know. However, his core values of friendship and protecting the village would still likely be there, just warped a bit by Zeref's dark nature.
Yes, there are likely some fanfictions out there with this concept. You can try searching on popular fanfiction websites like FanFiction.net or Archive of Our Own. Just type in 'Naruto Brother of Zeref' in their search bars and see what comes up.
The power - struggle theme is quite common. Each trying to prove their power is greater. Then there's the theme of self - discovery. As they interact, they learn more about themselves. And mystery is also a theme. There could be a mystery that only they can solve together in the fanfiction world.
If we consider the magical aspects of their powers, a Dragneel's power might be more raw and instinctive, while a Fullbuster's power could be more controlled. When they interact, the Dragneel's power could add intensity to the Fullbuster's ice spells, making them more powerful and long - lasting. Or the Fullbuster could use his ice to contain and direct the Dragneel's fire, creating a more focused and destructive force.
One thing that might make it unique is the combination of different fictional universes. Naruto and Etherious come from very different settings, so the crossover itself is distinct. The character of Dragneel could add another layer of uniqueness if he has a special role or ability in the story.
Well, I'm not sure how widespread it is, but it's possible to find such fanfiction. Writers often like to mash - up different universes to create something new. In a 'Harry Potter is Natsu Dragneel' fanfic, you might expect to see Harry with Natsu's determination and love for his friends translated into the context of the wizarding world. Maybe he'd be more brash and use his 'fire - like' magic in a more aggressive way like Natsu does.
A common plotline is the rescue mission. His friends from Fairy Tail band together to find and save him. Another is Natsu trying to break free on his own. He might use his fire magic in creative ways to escape his bonds.