If I were to make a game like Blame! I'd focus solely on exploration, survival and finding clues, keep a mystery going that you need to unravel.
Not much if any horror but a sense of dread at the scale and the mystery of the place.
Meeting interesting characters as a respite.
Not much focus on bing bing wahoo platforming or having too many "video gamey" mechanics because it would take you out of the game.
Sound and atmosphere most important.
Love and sadness very important too.

I guess I'd smash the best parts of Blame! with Made in Abyss.