For me, ideas generally come from "what if" scenarios. What if Joanna had been designed by W.A. Dwiggins, instead of by Eric Gill? What if Mozart had been a punchcutter—rather than a composer?
Compare that with an excerpt from Sid Caesar's account of how he and his colleagues wrote jokes for The Show of Shows (c1949), from his autobiography, Where Have I Been? (NY: Crown Publishers, 1982), p. 86—
We sat around tossing ideas back and forth, and we developed material of what we called the "What if?" category. Someone would say, "What is Christopher Columbus were an usher at Roxy Theatre?" and we'd take it from there, with Columbus navigating people to their seats. Or, "What if Leonardo da Vinci worked as a short-order cook?" and we'd have Leonardo sort of painting sandwiches together like each one was a work of art.
Or this by Robert Fitzgerald about a game he sometimes played with his friend James Agee, in Fitzgerald, ed., The Collected Short Prose of James Agee (NY: Ballantine Books, 1970), p. 23—
On one of these Sunday excursions when I went along I remember that we amused ourselves during the long black blowy subway ride by playing the metaphor game: by turns each describing an inanimate object in such a way as to portray without naming a public figure. Jim developed a second-hand flute into Leslie Howard, and a Grand Rapids easy chair into Carl Sandburg.