"The SF writer sees not just possibilities but wild possibilities. It's not just 'What if' - it's 'My God; what if' - in frenzy and hysteria. The Martians are always coming."
Philip K Dick, 1980.
SF as the rightful literature not of "What if - ?" but "My God; what if - ?!" is a motto I would willingly blaze in forty-eight point letters of gold upon every ideas folder I ever keep.
And I wonder how much of the spirit of modern fantasy can be well understood in such terms - not so much of "My God; what if this should come?", but of "My God; what if this should have been?"
To get from the idea to a story worthy of it, we then mostly need a protagonist who can answer, "By God, then this...!" - and to great triumph or tragedy, or occasionally even to great laughter, carry their answer and the reader all the way home.