New Mutants is Josh Boone’s take on the haunted house trope (inspired in part of by The Demon Bear Saga), and he told IGN the next two installments will apparently evolve the horror theme by examining different subgenres.
The folks behind New Mutants have very clearly framed it as a horror movie, but it is a specific type of horror movie.
Drawing inspiration from claustrophobic supernatural horror films like The Shining, the film will kick off a trilogy of Marvel-themed horror films that each bring a new spin to the scary superhero genre.