Let me start by offering a brief synopsis of The Roommate, courtesy of Wikipedia:
A college freshman, Sara (Minka Kelly), is randomly assigned to a dorm with a girl named Rebecca (Leighton Meester). They start off as friends, but things turn deadly as Rebecca begins to obsess over Sara's life, including her friends and family.A few things not mentioned here that pop up in all of the previews: the movie seems to be about Rebecca's attempts to take over Sara's identity. I have no doubt that is probably NOT a central plot point in this movie, but regardless it is what caused the wheels in my head to spin.
Obviously the idea is that Rebecca wants to become Sara for some reason or another. My thought was that instead of just obsessing to the point of stalkerish and law-breaking, what if Rebecca was really some sort of monster and was literally becoming Sara? She has some sort of monster magic and/or voodoo that erases her from existence, and assumes her new identity as Sara - ready to consume her next victim later on.
I guess my idea is sort of that Rebecca is some kind of creature that preys on other living beings and constantly changes identity to keep a low profile or whatever. I don't know. I just think that my idea is probably a lot more interesting than what the movie will turn out to be.