Looks great so far! - lighting especially which I'm guessing is better illustrated in a video.
One thing that pops into mind is that old fantasy movie with Tom Cruise (Legend?) - where they use a load of mirrors to reflect sunlight at a demon to destroy it... maybe that would be cool, like a mirror could become a light source and reflect the light back. Maybe some puzzles just have the player arrange mirrors to reflect sunlight from a window.
Ohh... another spitball idea...
What if you had these black figures in the shadows, maybe they give themselves away now and again with just a pair of eyes... but when you light the area enough they die, like in a puff of black smoke, if the player walks into one then it's curtains. Maybe some of these figures can only be killed by certain colours or types of light.
Anyway, I think we'll all be keeping a close eye on this, can't wait to see how it develops.
I am the one who knocks...