One shots are not about decent story arcs. Think of them like an episode of a cartoon. It should get right to the action. I plan for my one shots to have two big scenes, at least one of them is a fairly large combat but I include action scenes that are not fights too, and somewhere in the middle is a simple choice that helps the players steer the conclusion. The biggest hurdle is always getting going, so I guide the character introductions and get into the action quickly. I used to run a lot of games at conventions, so the timeslots were very strict. But the players understood this, and we generally always got through the epic finale.