Just changing a few starting conditions can yield fantastic results.
One of my favorite history scenarios is for the Chinese not to have turned inward, but instead had set out to creat and imperial presence in the Pacific Basin, as well as the Indian Ocean and Australia. The changes they could have brought to the Meso American cultures are amazing to think about.
Another is where Alexander the Great settles down in Alexandria, doesn't die young, and starts sending out explorations into Africa and West, ultimately into the Americas.
In that one, I see us colonizing the Solar System by the 16th century.
Ed