This book tells you what kind of movies have rarely or never been made and warns you about saturated niches. Authors and screenwriters might want to avoid those. The lower the frequency meter, the more unique the concept.
These statistics are based on about 3000 horror and horror-adjacent movies I've reviewed and rated. 30 subgenres and 8 formulas were combined, resulting in 232 permutations. The best movies for each subgenre/formula combination are listed to reinforce the writing prompt.