Story
Story is the most important place to start with when crafting your one-shot - after all, a one-shot is an interactive story in your world! It's helpful to think player-first when you're crafting your stories, and have a clear goal you want the players to strive for. This creates a clear resolution for your oneshot (rather than leaving it open ended), and keeps things short.
SOME STORY IDEAS
Here are just a few story ideas to get your started for your one-shot! The players are...
...on a rescue mission for an NPC?
...stranded survivors, trying to reach safety?
...solving a murder?
...searching for (or stealing) a lost artifact?
...investigating a strange occurance?
Hooks And Stakes
Remember to consider how you're going to hook your players into the story line. There's a bit more buy in with one-shots than with open-world adventures, but the plot hook has to be really clear, or your players will miss it.
And while you're at it, why not throw in some stakes, too? It might be as simple as life-or-death, as dramatic as the destruction of a town, or as pressing as a rising political coup. But it has to be clear what may happen to the players, or those around them, if they fail.
Challenges
A story without conflict is no story at all! What trials will your party face on the way? The specifics of challenges are strongly related to your genre, locations and story-type; but in general, try to mix it up between combat, social encounters, puzzles and exploration. You'll probably want to create some interesting monsters and NPCs for these, too!
The Setting
One-shots are a great excuse to create some juicy locations! For combat, pick places which will offer interesting challenges (maybe the floor is lava) and tactical advantages ("I have the high ground!"). For all locations, but especially social scenes and exploration, create spaces which dig into your genre and mood: grim-dark cyberpunk bars, taverns glistening with magical light, or a unicorn grove fluttering with fairies. Make it your own! You don't need a tonne of details here - flavour is the most important thing.
The Climax
This is the culmination of your adventure, and often the last barrier standing between the party and success! It's traditional to fight the big bad, in which case, introduce some interesting swarms or mooks, or other battlefield conditions, to make the fight memorable. But your climax doesn't have to be combat - you might have a dance off, play chess with death, or have an epic rap battle! And that's the key - whatever it is, it should feel epic!
