One common theme is the idea of different species interacting. In these fanfics, you often see wizards from Harry Potter, mermaids, and vampires from Twilight coming together. Another theme could be forbidden love, like a wizard falling in love with a mermaid or a vampire, which is similar to the love stories in Twilight. Also, the exploration of magic in new contexts, such as mermaid magic combined with wizard magic, is a frequent theme.
Yes, there are quite a few. You can find them on popular fanfic websites like Fanfiction.net or Archive of Our Own. Just search for 'Harry Potter Mermaid Twilight' in the search bar and you'll get a list of relevant stories. Some might be crossovers that combine the magical world of Harry Potter, the allure of mermaids, and the vampire - human love story of Twilight in interesting ways.
Love and attraction are also big themes. Just as in the original Twilight series, there are often love triangles or complex relationships. Maybe a mermaid falls in love with a vampire, but there's also a werewolf vying for her attention. It adds a new layer of drama to the story.
One common theme is the clash of different supernatural elements. Since Harry Potter has magic, wizards, etc., and Twilight has vampires and werewolves, seeing how these interact is a big theme.
One common theme is the clash of different magical systems. In these fanfictions, the magic from Twilight, like vampire powers, and the spells in Harry Potter often collide or interact in interesting ways.
One common theme is the exploration of different relationships. For example, fanfics often expand on the relationships between the vampires and the werewolves. Another theme is the alteration of character destinies. Writers like to play with the idea of what would happen if a character made a different choice.
One common theme is the struggle of the OC mermaid in a world filled with vampires and werewolves. It's about her trying to fit in or find her place.
One common theme is the exploration of Bella's identity. Since she's a mermaid in these fanfictions, it often looks at how she reconciles her new mermaid self with her human side. Another theme is the relationship changes. Her being a mermaid can greatly impact her relationships with Edward, Jacob and others.
One common theme is loyalty. Griphook's loyalty, whether it's to his own kind or to those he forms relationships with in the fanfics, is often explored. Another theme is greed, as Griphook is a goblin and the idea of his relationship with treasure and wealth is a recurring topic. And there's also the theme of trust, as in the original series, trust between Griphook and the main characters was a complex matter.
Adventure is also a big theme. Sebastian embarks on various adventures, like exploring hidden parts of Hogwarts or going on quests outside the school. These adventures are filled with magic, danger, and discovery.
One common theme is the contrast between Durmstrang and Hogwarts. Fanfics often explore how the more strict and sometimes darker Durmstrang differs from the relatively more open - minded Hogwarts. For example, in many fanfics, the Durmstrang students are shown to be more competitive and less friendly on the surface.