One main theme is the clash and融合 of different cultures. Harry Potter's world has its own set of magical and cultural norms, and the Red River world has its unique elements. When combined in fanfic, this often leads to interesting situations where characters have to navigate the differences.
Sure. One interesting 'Harry Potter and Red River fanfic' could be 'The Magic of Two Worlds'. It combines the magical elements from Harry Potter and the unique cultural aspects of Red River in an exciting way.
River Tam could be brought into the Harry Potter world as a sort of consultant. Given her knowledge and skills, perhaps Dumbledore summons her to help deal with a new and dangerous threat that has elements of both magic and advanced technology (like in her own world). She would then work closely with Harry and the others, sharing her insights and learning about magic in the process.
In fanfics, the red string of fate can be a creative device. For Harry Potter, it could be a magical string that only certain magical beings can see. It might show a connection between a wizard and a magical creature. In Twilight, it could be something that the Volturi are aware of and use to manipulate relationships. For instance, they could try to sever the red string between a vampire and a human to stop a potential powerful union.
In Harry Potter fanfic, the red string of fate could be part of an ancient magic. Maybe it's related to the founders of Hogwarts. For example, if the red string links two students, it could be a sign that they are meant to revive an old magic together. In Twilight fanfic, it could be a supernatural element from a different culture. A vampire might discover that the red string of fate connects them to a human in a way that defies their natural instincts.
One interesting 'Harry Potter River Tam' fanfic could be one where River Tam uses her unique abilities to help Harry in his fight against Voldemort. Maybe she can sense the dark magic in a way that the wizarding world can't and gives Harry an edge in battles.
Well, just based on that title alone, it's difficult to determine its quality. There are so many aspects that make a Harry Potter fanfic great. For example, it could explore new relationships between the characters, or create exciting new magical adventures. But we don't know if 'skull is harry potter fanfic' does any of those things. So, it might be good, it might not.
Character crossovers are essential. If a Red Lantern like Atrocitus were to enter the Potterverse, he could bring his unique view on justice and vengeance. This could contrast with Harry's more morally - complex view of fighting evil. Their interactions could drive a great story, as Harry tries to understand this new, more extreme way of dealing with wrongdoings.