Your heroine has been told that she should never meet a specific man. She is shown a picture of this man and told that if she ever sees him, she needs to turn and walk away as quickly as possible. If they meet, it is the trigger for the end of the world.
What happens?
Does she meet the man? Does someone try to force them to meet?
If they do meet, does the world end or can they do something to save it?
Does someone try to kill her to prevent her from meeting this man and the world ending?
So many possibilities!
Happy writing.