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.
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.
It could be a really interesting crossover. Natsu is from 'Fairy Tail' with his powerful fire magic and hot - tempered nature. Sakura from 'Naruto' is a strong - willed kunoichi. Maybe in the fanfic, they meet in a strange new world created by some magical or ninja - related event. They could start off as rivals, not understanding each other's powers at first. But as the story progresses, they learn from each other and combine their skills to face a common enemy.
One way would be through a magical portal. Maybe some dark magic in the 'Fairy Tail' world accidentally creates a portal that sucks Sakura into it. Once in the Fairy Tail world, she has to adapt quickly. Natsu could be the first to find her. He's initially confused by her ninja abilities but soon realizes they can be useful. They then work together to close the portal that's causing chaos in both worlds. In this way, the two worlds start to blend as they exchange knowledge and skills.
Well, in these fanfictions, the concept of the Etherious is really interesting. It allows for a lot of creative exploration. For instance, we can see different interpretations of how Natsu's Etherious nature affects his relationships with other guild members. Some fanfictions show him having to struggle with his darker side, which makes for a great read. Also, fans love to see Natsu in new and exciting situations that the original series might not have covered, like facing ancient evils related to his Etherious origin.