After filling in your question to What if?
Now is time to fill in the blanks.
I usually begin with characters. They are the prism in which your story is told. Who are your main characters? Who are your minor characters?
Knowing who they are makes it easier to plot. That is if choose to do so, more on that in another post.
Fully develop your characters. Really get into their heads. Question both your hero’s and antagonists motivations. Interview them. By understanding who they are, it will help to understand the story you want to tell.