In the demo version of ADVR there was a modifier called "Scared of the Dark". This would make the dungeon extremely Dark and added a horror and ambient element to the game that I absolutely treasured with all my heart.
I remember being disappointed about the lack of Scared of the Dark when I bought the game after playing the demo.
I think it would be a good hard mode addition, possibly different stages of darkness being different tiers of the modifier like how other hard mode additions work.
For anybody looking at the suggestion curious what I'm talking about, this is what Scared of the Dark looked like.