Hopefully it's not too late to add some suggestions;
- Something involving an AI going weird.
- Something that can possess creatures or corpses.
- Something that evokes the terror of either the deep sea or the furthest of underground caverns (ie; Something big... Very big, and very far down).
- Something that gives foreshadowing by stalking you between rocks in the background. Or wherever.
Anything else I think of... Eh. I dunno.