Some of the Death Eaters who might have survived or changed their ways in an alternate universe could be the slashers. They would bring their knowledge of the Dark Arts and their own twisted view of the wizarding world. Bellatrix Lestrange, if she had a different path, could be a very interesting slasher figure in such a fanfiction, as she is extremely powerful and unpredictable.
Well, a really interesting slasher crossover fanfiction could be the one that combines Chucky with Ghostface. Picture this: Chucky somehow gets involved in the Woodsboro killings and starts messing with Ghostface's carefully planned murders. It would be a wild ride of horror and dark humor. Also, there are some crossovers that include Pinhead from 'Hellraiser' with other slashers. These stories often explore how the Cenobite's otherworldly horror would interact with the more 'human' slashers.
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.
Well, it could be about a BTS member accidentally running into a gymnast YN at a sports event. Maybe they get to know each other gradually and BTS is inspired by YN's dedication in gymnastics. It could also involve YN getting a chance to perform at one of BTS's concerts, creating a unique and exciting plot.