I think there are two main types of 'What if?' questions. The creative, fanciful WI? questions and the WI? question that considers problems and even worst-case scenarios.
Creative WI? questions
What if...there was a million pound limit on personal wealth?
What if...there was one day a year when cars are illegal?
What if...all the Earth's water turned green?
What if...all the herbivores became carnivores and vice versa?
What if...people could converse only in song?
Problem WI? questions
What if...I was the only one that turns up for work tomorrow?
What if...all my possessions and money were stolen?
What if...I was arrested?
What if...I suffered from amnesia?
What if...all my family moved abroad?
However, I also like to create 'thinking the unthinkable' WI questions.
Thinking-the-unthinkable WI? questions
What if...I gave away all my money?
What if...I stopped washing?
What if...I tried to start a fight with everyone I meet?
What if...I started espousing everything I hate?
What if...I changed my name to Elvis Presley?
There are some good WI questions on this Innovation Tools article by Scott Ginsberg.