The 'power - sharing' trope is also common. This could be where Hermione's magic and the powers of the vampires or other supernatural beings in The Vampire Diaries interact in unexpected ways. Maybe she uses her magic to enhance their powers or vice versa. It creates a lot of interesting plot possibilities.
The 'time - travel' trope can be seen in some fanfictions. A character from 'Vampire Diaries' might travel back in time to the era of 'Merlin' and change the course of events. Or vice versa. The 'hidden identity' trope is also popular. A character might be hiding their true identity as a wizard from 'Merlin' in the 'Vampire Diaries' world. And the 'battle of powers' trope, where characters from both series test their powers against each other to see who is stronger.
One common trope is the outsider - within - Slytherin. Hermione is smart and different from the typical Slytherin, so she has to find her place. Another is the redemption of Slytherin as a whole. With Hermione there, she might start to change the house's more negative aspects. There's also the friendship - turned - rivalry - turned - friendship again with characters like Draco. They start off on the wrong foot, become rivals, but then realize they can work together.
The 'Forbidden Love' trope is also common. Kai might fall in love with a character he shouldn't, like a vampire hunter or a witch from an opposing coven. It creates a lot of drama and tension in the story. For instance, in some fanfictions, Kai's love interest is from a family that has been sworn enemies of his for generations, and they have to fight against all odds to be together.
One of the most popular tropes is the 'forbidden love' trope. Since Stefan has a complex past and Caroline has her own set of responsibilities in the vampire world, their relationship can often be seen as something that's not supposed to happen. It adds a lot of drama and tension to the stories.
One popular trope is the 'fix - it' where characters go back in time to change a tragic event. For example, in many fanfictions, a character might go back to prevent the death of a loved one like Jeremy. Another common trope is the 'fish - out - of - water' situation. When a character travels back in time, they have to adapt to the different time period, like different social norms and fashion. This can lead to some really funny and interesting situations.
One popular trope is the 'character swap'. For example, having Harry Potter end up in the 'Vampire Diaries' world and having to deal with the vampire politics there. It's interesting to see how his Gryffindor courage plays out in a different setting. Another is the'magic - mix' trope where the magic from both universes combines in unexpected ways. Like using a spell to control a vampire or a vampire's blood having some sort of magical effect on a wizard.
One great Hermione Granger and Vampire Diaries fanfiction is 'The Crossover Chronicles'. It combines the magic of Hermione's world with the supernatural elements of The Vampire Diaries in a really interesting way. The author does a great job of making the characters interact in a believable setting.
In many 'Training Hermione Granger' fanfics, the 'training montage' trope is quite popular. It's like in movies where you see a series of scenes showing Hermione getting better and better at her skills over time. Also, the 'forbidden knowledge' trope often appears. Hermione delves into magical knowledge that is considered dangerous or forbidden, and her training is centered around handling this knowledge without getting corrupted. Another trope is the 'team - up' trope, where Hermione teams up with other characters, perhaps some from other magical schools, during her training for a common goal.
One popular trope could be the 'outsider' trope. Pearl and Jonathan might be seen as outsiders in the vampire community, and the fanfiction could revolve around them trying to find their place. Another is the 'redemption' trope, where they might have done something wrong in the past and are now seeking redemption.
One popular trope is the 'enemies to lovers' scenario. In both Charmed and Vampire Diaries, there are characters with opposing views or allegiances that fans like to pair up romantically. For example, in Vampire Diaries, a fanfiction might explore the relationship between a vampire and a vampire hunter who start off hating each other but gradually fall in love. Another trope is the 'alternate universe' where the story takes place in a completely different world from the original shows, like a world where the witches in Charmed have different powers or the vampires in Vampire Diaries don't have to feed on blood. And the 'time - travel' trope is also common, where characters go back in time to change events in the shows' histories.