A good blend of the two universes' mythologies. In a Buffy Supernatural crossover, it's important to merge the different supernatural elements in a way that makes sense. This could be how the vampires in Buffy's world compare to the demons in Supernatural, or how the magic works in both. It should feel like a natural combination, not just thrown together randomly. The plot also needs to be engaging. It should have a clear goal or mystery that draws the reader in and keeps them interested throughout the story. Maybe it's a new threat that only the combined forces of Buffy and the Winchesters can face.
Good writing style also matters. The language should be easy to read and flow well. If the fanfiction is full of grammar mistakes or is written in a really convoluted way, it's going to be hard to enjoy. Additionally, the fanfic should add something new to the existing universes. It could be a new relationship between characters from the two shows, or a different take on a well - known supernatural creature. This makes the crossover feel fresh and exciting for fans of both Buffy and Supernatural.