More necromorph variety
In Dead Space 1 especially, pretty much every enemy could be defeated in the same manner - slice off their legs and then an arm or two: there, dead.
Dead Space 2 did make an attempt at introducing new enemy types to force the player to mix up their strategy a bit. The Pack were pretty decent, as were the Stalkers who would sneak up on and flank the player. The regenerator necromorph also made for a great change of pace, as mentioned before, but we wouldn't be averse to a little more variety.
Surely humans aren't the only living creatures in the future, so how about some animal-based necromorph? Kind of like how the xenomorphs in the Alien films are different based on what species they cross-breed with, resulting in creatures like the dog alien from the third film. This could give the necromorphs more wildly varied behaviour patterns, and could mix things up quite significantly.
Also tying in with making the game scarier, the variety could come not just from the monsters themselves, but from where they spawn. Basically, when you've finished the game once or twice you learn when and where all the enemies are going appear. By adding in a slightly random element to their spawn points, it could keep gamers on their toes no matter how many times they play. The game could still retain the scripted moments when it needs to, but for the most part, random enemy spawns would be more terrifying.