6. Cloverfield II - Matt Reeves
The Movie: The ideas remain vague, but one of the most commonly suggested plots is an alternate perspective of the events in New York, perhaps from those who live through the final moments of the first film.
Why It Hasn't Been Made: Writer Drew Goddard, producer J.J. Abrams and director Matt Reeves are all busy, in-demand folks, and that's the prime reason. Goddard is trying to write Robopocalypse, Abrams has Star Wars Episode VII and possibly Star Trek III, and Reeves is directing Dawn of the Planet of the Apes, so there's a lot to get out of the way before the three can get on the same page, even if they have reportedly been floating ideas by one another. Also, in an interview, Goddard stressed that Paramount haven't been breathing down their necks to force out a sequel, instead giving them time to work out all of the finer details, so hopefully we'll eventually get a sequel worthy of the excellent original movie, but just not for a while.