After seeing it I thought to myself, "This film is a fantastic standalone film - there's not going to be anything else". I've thought it about films such as Hancock and Cloverfield, too, and it saddens me that the industry is in a phase where it feels the need to constantly produce franchise films. What I'd like to see more of is films like D9, Hancock and CF - films that are single films created from completely new material (not an adaptation of a book, or a comic, or a remake of an old film). Sure, they're all good in their own ways, but they just can't compare to something fresh.
...... You really don't think there's going to be a sequel to this movie? They left it wide open for a District 10.