I'm starting to dry up on IP's that would be truly 'holy cow' at this point that haven't appeared yet. I mean, most of the biggest hitting horror franchises have been represented at HHN at this point (Halloween, Nightmare, Friday, Saw, Texas Chain Saw, The Exorcist, The Evil Dead even if it was based on the remake, Chucky, American Werewolf, The Thing... the list goes on.)
Poltergeist (the original perhaps) might surprise some folks... Perhaps even a stand alone Alien maze based on the first or first 2 films. The only big horror classic I could see truly being a shocker is The Shining considering rumors of HHN trying to work with King in the past to no avail.