There aren't too many movies that necessitate sequels. Unless a movie is part of a pre-proposed series or is an adaptation of a series of books, it should probably be able to stand alone. But a lot of sequels come from movies that are perfect by themselves -- sometimes the sequels compliment nicely; sometimes they are easily ignored; occasionally they actually take away from the previously regarded original.
It isn't often that a movie screams out for a sequel, but I think I've come up with seven that at least whisper a request for one. Two actually have source sequels that they would be adapted from. One has a lot of history to mine material from. Three of them have been discussed at length at different points in time by makers of the original(s). The problem is that none of these sequels is likely to ever grace your DVD player let alone your local theater. For whatever reason, they simply have too much against them in the minds of studio execs. For now, though, we can dream.
1. The Restaurant at the End of the Universe (sequel to The Hitchhiker's Guide to the Galaxy)
Even with the incredible cast and the surprisingly faithful-enough script, The Hitchhiker's Guide to the Galaxy was not the epic that I was hoping for. It also wasn't the blockbuster that Disney was hoping for. The filmmakers, Garth Jennings and Nick Goldsmith (aka Hammer and Tongs) and the necessary actors had signed on for the sequel, to be adapted from Adam's follow-up, The Restaurant at the End of the Universe, but it appears to be dead in the water. Despite my few reservations with the first film, I would love to see the sequel, as well as the rest of the series (they could end before The Salmon of Doubt, I guess). I remember being bored with some of the prehistoric Earth sequences in Restaurant, but I think they'd make for great cinema. In any event, I think Martin Freeman and Mos Def were a great duo in the original, and they alone should have been propelled to stardom following its release. Maybe they can appear in something else together.