I'm a big fan of the alternate history genre. Even mediocre authors, such as Turtledove and Harrison, can produce compelling works. I would estimate that about 50% deal with the American Civil War, which gets tired after a while. Here are a few senarios that I would like to see written:
1. June, 1945: war breaks out between the U.S. and the Soviet Union in Germany.
2. 1492: Horses came over the Bering Strait ice bridge when the Native Americans arrived thousands of years before. Consequence: Mesoamerican civilizations are very technologically advanced, on par with the late Roman Empire.
3. 1863: In the midst of the Civil War, Utah Mormons successfully fight for independence.