I used to base stories around something like this: What is a well known aesop or moral known to you. Does it have an exception, now write about that exception.
It was when I was still experimenting with survival horror that I originally formed a story about the exception to the aesop "thou shalt not steal", with the natural theme sort of coming out that being about the sin of greed. Sort of an anti-aesop methodology.
And then usually I base it on an experience. Not a concrete experience memoiresque experience, but rather a sense of time or place I'm familiar with.
I'm not sure why I don't do that anymore. It used to be easier to get longer stories done that way as I didn't think about have linear plots as much as I do now.
Does anyone else have a formula they fall back on, if they don't outline? How has it worked for you?