Aha. Showing some more pieces did indeed help a bit in understanding the problem. I think what is probably best is that you work towards a mockup. What this means is a full scene that represents more or less what the game looks like.
You already made a bunch of survival characters, that give a good idea of what the people will be like (leaving style alone for a minute). You already expressed having issues representing horror (don't worry, we can get back to that later, I think I have a few cool examples for you that would fit your style just fine), so we'll leave that last.
So, the environment, what's that going to be like? You could start pixelling it, but you could also take some random google image to describe a feel you get. You could even make some sort of collage out of these (I could show you how to, if you want). Next you should place your characters into this scene and start playing around until it feels like what you want. Once the characters are in a scene, it's going to be easier to gauge what is "childish", what is "too plain" and where to take it from there.
Sound good?
