The default state of horror cinema is one idea stretched across a hundred entries. Big franchises seem to go on forever, well-contained stories suffer sequels they can't carry, and the world gets sick of ideas that they used to love. Horror filmmakers love sequels so much, they'll sometimes suggest one that may be impossible.
The ending of a horror movie can often feel a bit by the numbers. Either the final girl gets away and the world goes back to normal, or the killer gets away and the rest of the cast inhabits shallow graves. One extremely common way to spice things up would be to dramatically reveal that the killer is still alive and well. Obviously, spoilers ahead for the somewhat anticlimactic conclusion of everything mentioned.
It Follows
Of all the sequel teases that lay unfulfilled, this seems like the one that has the highest chance of paying off. This 2014 instant horror classic rocketed its director David Robert Mitchell to the upper echelon of up-and-coming filmmakers. It Follows follows Jay, a young lady who finds herself the victim of a sexually transmitted paranormal entity. The titular It pursues its prey at a snail's pace and only stops when the target adds a new name to its list. Despite its apparent invulnerability, Jay and her friend Paul trap it in a swimming pool and shoot it repeatedly through the head. Though It appears to be dead, a figure can be seen behind Jay and Paul as they walk down the street. Mitchell has mentioned his desire to pursue a sequel in which Jay works backward to find the origin of It, but nothing has come of it. As it stands, the story ends there.
Slither
Though he's one of the most well-known creatives in the world of cinema today, there was a time when James Gunn was just a quirky little director making weird gross horror movies. His directorial debut, Slither, came out in 2006 and enjoyed rave reviews from critics. The film covers the nightmarish incursion of an alien parasite into a small South Carolina town. The film has the structure of a modern Resident Evil game with the increasing scale of arms intact. The battle with infected aristocrat Grant Grant ends in a massive fireball, reducing the town's population to only three survivors. The trio takes to the woods and the credits roll. Unfortunately, a post-credit scene reveals a cat infected by the deadly parasite. The infection is loose and the whole world is at stake, but we'll never know how that went.
Freddy vs. Jason
The clash between the two most iconic slasher villains of all time had a fair amount of tease before its release. As fans know, the end of Jason Goes to Hell: The Final Fridaydepicts Freddy's clawed hand dragging Jason to the depths. A full decade later, that tease was paid off with Freddy vs. Jason. Fans got all the silly conflict they could've wanted out of that film, but it still had to end on a cliffhanger. The group of generic teens who are embroiled in the bout finds themselves rooting more for Jason than Freddy. With some help, Jason decisively wins the duel, leaving Freddy nothing more than a disembodied head. Of course, that head then winks to the camera, proving that the Springwood Slasher is still on the loose. They've never battled on the big screen again, but there are some comics on the subject.
Truth or Dare
This 2018 Blumhouse production proves that not every tease needs to pay off. While some teases are left unfulfilled by lack of budget or changes in leadership, this film was simply despised by almost everyone who saw it. The film depicts a game of Truth or Dare conducted by a seemingly all-powerful demon who breaks his own rules at almost every turn. The game only matters when the script can be bothered to remember it. At the end of the film, the teens trick the demon into telling the unfortunate truth that they have no way of leaving the game. Their only solution is to add players, which they accomplish by spreading the game via YouTube video. This could throw the world into chaos, and director Jeff Wadlow has mentioned some sequel ideas, but it hasn't come up since. Hopefully, the game really is over.
Behind the Mask: The Rise of Leslie Vernon
This horror mockumentary covers all the tropes of a classic slasher film, so it simply must end with the killer suddenly returning to life. This 2006 film was something of an indie darling, but it didn't make much at the box office. Writer/director Scott Glosserman has spoken multiple times about his desire to make a sequel, but the story keeps changing. Over the past 17 years, the concept probably hasn't aged well, but there's still a lot of fun to be had with the idea. Like Freddy vs. Jason, fans can find the answer in comic book form if they'd like.