Last evening (4/8/14) I heard about the
Holocene Impact Working Group. This leads to several interesting ideas. Example: Remove the 535AD
Event. The events or event (it's a big mystery) changed things on all inhabited continents. Remove those changes and the world looks radically different.
Another way to go is to introduce cometary impacts at key points in history. Smash Western Europe with a dozen comets in 1750AD. The Americas go off in their own way (the Anglophone settlements had the population and technical skills and would be way ahead). Asian developements would probably be slow or stagnation. PCs could be Americans exploring the ruins of Europe, Jihadists seeking to find and conquer "
the Golden Apple," or Europeans trying to rebuild their lands or escape to the Americas.
Ken Hite created Lucifer-5 a parallel Earth that divereges when the
Tunguska Event happens over St. Petersburg, instead in the wilds of Siberia. Even without the Space Opera goodies collapsing Russia in 1908 creating a Kaiserine Empire streaching from the Rhine to the Pacific would create a grand Kalter Kreig campaign.