I was going to make this about branching storylines, but the title for that inspired a different and much simpler game:

Your character is going somewhere, (where?) for a reason (why?), but they make just one simple mistake (what?) like a left instead of a right, missing a turn off, getting on the wrong train. Just one mistake. What does that one mistake to do their trip, what does it become instead?

Hint: this is the starting point of a plethora of mysteries, horrors and crime novels. Think big.