Plot twist, in dead space 4 we will be playing as necromorph. The story will take places 100000 years after events of dead space 3.
It will turn out that what we seen in dead space 3 was literally a birth of new spiecies. Just like humans came from aggressive, barely intelligent and primal Neanderthals, what we seen in dead space trilogy was a primitive and undeveloped form of "life" coming to life.
After all that time, the necromorphs now turned into fully sapient and intelligent spiecies, developed own culture and civilization but new threat emerges as a race of beings from distant galaxy stumbles upon world of necromoprhs and is dead set on wiping them all out seeing them as "monsters" and "atrocities".