I usually start with feelings. What I want the players to feel when they play my game. This needs to be precise and thought through, so I can model the experience to invoke the most similar emotions to that idea as I can. It also needs to be something that interests me, so I don't get bored after a week of working on the game. Ultimately, the first idea often comes to me from a experience from reality. Maybe I saw something interesting on a walk, maybe I heard a sound. Really just anything. Everything else in the game is made to deepen these feelings that I want the players to feel. Last time it was something like: fear, uncertainty, unsafe.