Having a, author/scriptwriter clever enough to write a book/script about a new creature to exist in the realm of vampires, werewolves, etc: then having someone willing enough to finance said project, bearing in mind we are talking about $10s of thousands of dollars through to millions of dollars. The financial benefits are there but only if people fund those benefits.
What I do is without much cost (Fan Fiction) but getting a high readership is wishful thinking when writing a new story.
What I'd like to see in the new Buffy is a completely new variation of vampire/ werewolf.
Plus who is the new slayer going to be, Buffy or Nova? Personally I hope it is Nova, the person who Sarah Michelle Gellar said it would be.