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.
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 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.
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 don't think it's a common concept. While both Harry Potter and Natsu are popular in their own right, their worlds are quite distinct. The Harry Potter series is centered around a school of magic, magic spells, and a battle against the dark forces. Natsu, on the other hand, has his own set of adventures in his fictional world. So, not many people might think of creating a fanfiction that combines them.
One common theme is the exploration of relationships. For example, romantic relationships between characters that were not fully explored in the original books, like the relationship between Harry and Hermione. Another theme is the expansion of the magical world, such as new spells or magical creatures. Also, there are fanfictions that focus on the backstories of minor characters, giving them more depth and significance.
Love and sacrifice are also prevalent themes. We see how the love of Lily Potter saved Harry in the original series, and fanfiction often expands on this idea of love being a powerful force. There are also fanfictions that explore the idea of redemption for characters like Draco Malfoy, showing that even those on the wrong side can have a chance at changing and making amends.
One common theme is adventure. Combining the action - packed world of 007 with the magical adventures of Harry Potter makes for some thrilling stories.
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.
Love is a very common theme. Their relationship, from the initial attraction to the deep, abiding love they share as a married couple. Another theme is family. Fanfictions often explore how they raise their children and the Potter family values.
Common themes include the exploration of Hogwarts in different eras. Since James was at Hogwarts before Harry, fanfictions might compare and contrast the two times. For example, the different social dynamics among students, the teaching methods of the professors. Friendship is also a big theme. James had his group of friends, the Marauders, and Harry had his own friends. Fanfictions may show how these friendships are similar or different. Additionally, the fight against the Dark Arts is a common theme. Both James and Harry faced dark wizards, and fanfictions can explore their different approaches to this fight.