-Even though it hasn't been long since you started, this demo already has much to offer. I didn't encounter anything that broke the immersion, rather the opposite. Some encounters are intimidating in a fun way (like that first scarab with glowing eyes you come across 5 min into the game)
-There's something very comfy on the way how things are designed and it works well together. It's a game which is overall fun to explore.
-Sometimes the mist reminds me of the darkness on Tenchu 1 and 2, which at that time was a limitation of the engine, but it created a gorgeous atmosphere)
-Since atmosphere what I liked the most, I think adding sounds and music that enhance it would be great.
I'd love to see how far you can go with this. Good luck!