This is a great question.
Where do you get yours?
I often quote a piece of Warren Ellis bloggery where he tells us that part of the job of being a writer "is to stare off into the distance to think about what's next." Okay, it's not really a quote - more of a paraphrase but it gets the job done. It worked well enough in my day job and it's really true.
That's why us writer-types like windows in our writing spaces, as I mentioned on Monday. We need to be able to unfocus our eyes, let the brain go wild and roam about the countryside of our imagination. The input that may or may not be with us (and I'm thinking primarily of auditory input: music, TV, sounds of nature, etc...) is going to help us find the 'happy place' where ideas are gathered in herds, grazing the fertile fields of the possible.
If you've read anything about the creative mind, you've come across similar things that are equally ludicrous when you think about it. But to take that metaphor just a little further, the fields that ideas are grazing in are filled with grasses that are actually our own experiences. That's the fuel that fires the imagination. That and asking one of the two most important questions in the world:
Have you ever thought about --- ?
Which are really just variations of one another but for me they spur different patterns of thinking.
"What if" is the beginning question. The one that spooks the herd and gets it moving, like the gallimimus flock in Jurassic Park.
"Have you ever thought about ---" is the leading question that I have to ask myself once I've gotten into the story. Even the characters will ask themselves this sometimes. Once I've got a starting point, asking this one becomes more important because it can then alternate with "What if" at every stage of development of story and character.
It also works in real life for a day job as you're considering decisions that must be made.
So this strange intersection of "What if" and "What then" are connected not just by their beginning words but also by the word "and" in this particular case. This word can also function as a question of the leading variety. "This happened," I write, then I write some more. "This happened next then all of a sudden this happened." (As an aside, I never use the phrase 'all of a sudden'.) So now I'm stuck.
Then what? Have you ever thought about ---?
See how that works?
So if you ask me where I get my ideas, I'll tell they show up because I ask questions. Or they're buried somewhere in a stampeding flock of bits of things that charge around the veldt. Or the prairie. Whatever.
Use your imagination.