For one-shots, sounds like you just need a basic outline of a story to throw at players? I sometimes find random story idea generators (for RPGers and/or for writers) helpful for inspiration, though I don't use them so much anymore.
Most of the following tools just cobble together a sentence or two plot outline, basically a short pitch for a short story. The first site listed is fairly modern, and has a section called "Blurbs" that let you enter specific story elements if you already have certain genres, character concepts, setting elements, etc, in mind (like "location is an abandoned arctic oil field" or "protagonists are mercenary android hunters" or whatever). The other sites have been around a while, include tons of other generators of various sorts, and may or may not have suffered link-rot over the years.
These are ones I've referenced before and happened to still be in my bookmarks. A search on something like "plot generator" will likely net you many more, including ones that are more modern in design and perhaps better supported.