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.
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.
A theme that is quite prevalent is the idea of sacrifice. In many fanfictions, either Natsu or Lucy is put in a position where they have to sacrifice something for the other. It could be their own safety, their magic, or their dreams. Also, the theme of growth is common. They both grow as characters, learning more about themselves and each other as the story progresses. This growth can be in terms of their magical abilities, but also in their emotional maturity.
One great 'etherious natsu dragneel and lucy fanfiction' is 'A Bond Beyond Magic'. It beautifully explores their relationship in a world where they face new and unexpected magical challenges together. The way it develops their characters and the unique magical situations they encounter make it a very engaging read.
I really like 'Etherious Natsu's Adventure'. This fanfic takes Natsu on a journey where he has to face some of the darkest secrets of his past as an Etherious. It's well - written with a lot of action and emotional moments. For example, there's a part where Natsu confronts a figure from his forgotten past, and the way it's described is really engaging.
Well, a common theme might be the idea of the underdog. Harry was an underdog in the wizarding world at first, and Natsu is also someone who has to fight hard against strong opponents. In these fanfictions, this theme could be explored more deeply. Also, the concept of family could be a theme. Natsu has his own idea of family within Fairy Tail, and Harry has his friends as a sort of surrogate family. The fanfiction could blend these two ideas together.
One possible fanfiction could be 'The Abduction of Natsu' where a dark wizard kidnaps Natsu to use his power. Another might be 'Natsu's Captivity' which focuses on how his friends try to rescue him. There could also be a story like 'Kidnapped: Natsu's Ordeal' that delves into Natsu's own struggle while being held captive.
One interesting crossover could be a battle scenario where Natsu's fire - based powers from Fairy Tail are pitted against the devils in Highschool DxD. His dragon slayer magic might give him an edge against some of the lower - level devils.
Well, it could be about a girl who has to choose between Natsu Dragneel and someone else. Maybe she meets Natsu in a magical adventure and gradually falls in love with his brave and warm - hearted nature.