Yes, yes, the oldest writing prompt ever. But why? Because anything could happen. A character opens a door, and something inevitably happens. Even if the “bar,” or whatever place the person is entering, is empty—well, that’s a story, too. And opening a “door” whether physical or metaphorical, means change.
Open your manuscript to the page you’re working on now. Make a list of ten “doors” your character could open. Into a new house? Their old house. Their office? A school? The wrong room? The police station. A child’s room. An airplane.
What might happen? How would they react? And most important, why? How would that change your story? Does your character need a new door?