Prequels have been part of our cultural landscape recently. Here's where we get the chance to imagine as-yet unwritten prequels to well-known books! (Obviously, not all prequels deserve to see the light of day...)
And now to begin...
H. G. Wells' Chamberlain goes to Mars
Hemingway's The middle-aged man and the sea
Homer's The Helenad: Helen's dating days
The Bible's 10 minutes before there was light.
J. Fennimore Cooper's Next-to-last of the Mohicans
H. Melville's The misadventures of Ishmaël on a farm, his dealings with the deacon's daughter, and the reasons that prompted him to take to sea.
Let's hear some more ideas!