Zombie apocalypse alternative:
First thought, think Silent Hill.
There is a bizarre world wide incident. There are sporadic shifts in reality, where people are transferred to a shadowy mirror realm of the world. In this world, there are two commonly known types of individuals. Those who are unaffected, and those who are driven by their id. The id driven are little more than monstrous maniacs, acting out their fantasies and obsessions without restraint. Within a month, the worlds population was reduced by a third and it continues to decline.
Worse still for those who maintain their sanity, the ids have learned about the sane and their immunity in the real world. Some are merely envious,some actually hate them. These feelings of animosity have caused the ids to actively hunt the sane in the shadow realm, regardless of their feelings in the real world.
Second installment, the dream world is beginning to take shape, the dead exist in sporadic ghost-like forms, bizarre creatures wander about, trees wave to and fro when there is no wind.
For the third installment, there are the dream weavers, people who can create beings and environments from their imaginations, allowing them, and those around them, to live out their dreams in the shadow realm. Some are rumored to be able to pull people into the dream realm without the global shift.
Possible modifications, make it about the id, ego and super-ego.