When I’m stuck, world-building is usually my go-to.
(Is there some taboo about talking about game specifics? People never do, so please let me know if I’m violating the Unwritten Code.)
One of my shelved ideas was essentially a post-apocalyptic mystery. That was it. All I had, except for some specific imagery involving visiting a house and having it be all overgrown and creepy. I knew I wanted a sense of isolation, that travel was tremendously dangerous. That was it.
You could kind of start anywhere with that - what’s creepy about the house, or what the main character would be like, or a scene I wanted to have. I could have started with game mechanics - anything special or unique. Instead, I thought about what would cause isolation, not just for the main character, but the world. I wanted a place without a sense of “safe”, so nothing like a space station where the main character just needed to get to Earth and it would be all okay. Supernatural creatures could do it. Hordes of vampires or zombies. Constant dragon attacks. Or it could be something more mundane - radiation outside, or disease. I liked the disease idea, since I’ve got a pretty solid biology background, and went with fungus because you can see it, and fungal diseases can be seriously creepy. I thought about thrush, and posited a fungus like that, only faster and meaner. An open wound would be completely colonized with fuzzy white stuff within hours. Breathing the spores would cause quick death.
It would make sense for a population to stay contained and sterile.
From there, it gets easier. Where would people live? Would some places be safer than others? Would dry, hot Arizona be a haven for the wealthy, while Latin America was taken over? What kind of government would pop up? Who would grow food? Who would do the investigating of crimes - it would have to be someone outside the community, but someone who’s taking tremendous risks by traveling outside. What sort of things would become taboo (eating mushrooms, say)?
Once you have a world, then it makes it easier to move the character through it, and it may shape the story. Once I had a sense of the world, I knew I wanted the opening scene to emphasize the dangers of the outside vs. the sterility and perceived safety of the inside, so it was a scene where the character entered an outpost. This was an automated but complicated process involving bleach, verifying identity, destruction of contaminated items, etc. From there, it was a matter of building a realization that the inside, while apparently safe, was just as dangerous as the outside, which became easier as I understood what was scary in that place and what wasn’t.
If world-building isn’t a part of your WIP, you could do the same sort of thing with plot or character or mood. That is: what’s compelling about your idea? What would accent those parts, or contrast against them? How do you want the player to experience the story?