How do you decide on what is going to happen in a game?
My advice is that all events on a timeline come in three varieties:
Optional: The possibility of it happening, the outcome and the scope are all determined by the actions of the players. These should be the most common and tell the day to day story of the world. Examples include visiting a merchant and making a deal, attacking a goblin cave, or even something as simple as giving a beggar some coin.
Variable: These events are destined to happen but the players can affect the scope and scale of the outcome. These should be the events that drive the plot of the story. Examples include a hobgoblin group assaulting the town, a large scout group sited as a vanguard to an invasion, or the mayor asking the characters to investigate some ruins.
Fixed: These should be the rarest of all event as they are outside of the players ability to have any affect upon. Examples include meteor strikes, anything in the history of the world, the empire falling due to the king being assassinated half a continent away.
This formula will aid to keep the game on track, keep the players feeling in charge of their fate as well as engaged.