My response would depend on what you wish to focus on:
a) story-telling/character development
b) grim resource management, tactics, and strategy
c) exploration of setting (and also, do you have a setting in mind?)
However, I tend to be a bit of a system Hacker, so "write an appropriate hack of X" might be my first response. (Where X = {Apocalypse World, Fate, Savage Worlds, Archipelago, etc.)
I haven't played Blades in the Dark, yet, but it looks interesting.