Neill Blomkamp, the director of District 9, was all set to direct Alien 5 just a couple years ago. The film would have served as a direct sequel to Alien and Aliens (a.k.a. the films everyone likes), ignoring all the other sequels. But when Blomkamp's Chappie tanked at the box office, the studio decided to commit to Ridley Scott's trippy prequel series instead.
After the success of Freddy vs. Jason, the question remained... who could they fight next? Freddy vs. Jason vs. Ash (the hero from the Evil Dead series), was the logical answer. A treatment was produced but Bruce Campbell didn't want to share the character, and the studio behind Freddy Krueger allegedly wasn't happy that their flagship character would get his butt kicked twice in a row. But the treatment was eventually turned into a very good comic book, which you can read right now.
Tim Burton's Beetlejuice was a runaway smash hit, a wholly original horror comedy about workaday ghosts. A sequel seemed inevitable, and it was allegedly in the works for years under the title Beetlejuice Goes Hawaiian. Tim Burton allegedly loved the idea of combining German Expressionism with a surfing movie, and the film would have followed the Deetz family as they traveled to Hawaii, and featured Beetlejuice himself winning a surf competition.
The first two films in the supernatural superhero Blade franchise were big hits, but not quite big enough to make the third film the way writer David Goyer wanted. Goyer proposed a third film (we'll just call it Blade: Apocalypse for short), set in a world where Wesley Snipe's character was alone in a world entirely populated by vampires, a la I Am Legend. The studio balked at the price tag, and the cheaper, lamer Blade Trinity is what we got instead.
It's hard to believe that it's been eight years since the last Friday the 13th movie, a franchise that used to pump out sequels practically every year. But it's not for lack of trying. Recently we came pretty close to getting a Jason Voorhees slasher movie done in the found footage style, a format that was so played out by the time the F13 filmmakers thought of it, the fan backlash was so enormous that they backed off. The series is still stuck in limbo, but if Friday the 13th: Found Footage was all the producers had, maybe - just maybe - that's for the best.
Another idea that emerged after the success of Freddy vs. Jason was a mash-up of the Halloween and Hellraiser franchises, a concept that probably made more sense back when the serial killer Michael Myers was part of the supernatural Cult of Thorn (a plot point that has long since been abandoned). The studio wanted the crossover to happen, and wanted Clive Barker to write the screenplay and John Carpenter to direct, but the producers of the Halloween series rejected it, and the project never came to fruition.
Another Hellraiser idea that never quite can together was Hellraiser: Hellfire, which would have been the fifth film in the franchise if Bloodlines hadn't dried up. The ambitious storyline would have been about a businessman who made a deal with Pinhead, and used the Lament Configuration to bring about a Lovecraftian apocalypse. It sounds like a great idea. Instead we got a seemingly endless series of cheap, half-baked straight-to-video sequels with little-to-no relation to the original concept. Sigh...
The cult classic The Monster Squad wasn't a big enough hit to warrant a sequel, but it probably should have been, since the idea of badass kids fighting monsters could easily have spawned a big franchise. And the plans were BIG. Fred Dekker once revealed that the only idea he ever had for a follow-up was Monster Squad vs. Godzilla, which sounds like the best idea ever. Kaiju monsters have a long history of teaming up (and trying to kill) little kids, so throwing The Monster Squad into the mix was about as natural a crossover as you can imagine. Except for other Kaiju, of course.
Before he became the Oscar-winning director of the Lord of the Rings movies, Peter Jackson was an up-and-coming horror filmmaker who had a pretty neat idea for a film called Nightmare on Elm Street 6: The Dream Lover. The proposed sixth film in the series would have found Freddy Krueger beaten up nightly by Elm Street kids who don't think he's scary anymore, a pretty straightforward commentary on how jokey the franchise had become. Freddy would have had to become his old, frightening self again to gain the upper hand. The studio didn't go for it, but the idea of Freddy reclaiming his horrifying persona did become an integral part of Wes Craven's New Nightmare, and New Line liked Peter Jackson so much they eventually went into the Lord of the Rings business with him.
The blockbuster success of the serial killer thriller Se7en would normally have led to a sequel, but the film's seemed to preclude one. But the studio tried anyway with a film allegedly titled Ei8ht (yes, really). The film would have been adapted from a previously existing screenplay called Solace about a psychic who helps catch a serial killer, because yeah, that sounds like Se7en. The sequel never saw the light of day but the script for Solace did eventually get made into a film starring Anthony Hopkins, Jeffrey Dean Morgan and Colin Farrell.