“Islamistan” is the working title for a new timeline idea of oshron's in which Islam is the most common religion in the world (rather than Christianity), with its point of divergence somewhere in the Middle Ages. The actual “Islamistan” is a large number of Muslim-majority countries (which are not unified) which stretch all across southern Europe, the Middle East, southern and Southeast Asia, and across the Pacific into the Americas and back to Africa. The other major powers ITTL would be Christian-majority Sweden, which establishes the largest European empire outside of the Old World; Russia, whose tsars adopt Islam rather than Christianity but eventually adapt it into something different; and a Buddhist Mongol empire stretching from China to the Arctic. Europe outside of the Muslim-majority states becomes TTL's version of the Middle East, a less-technologically-advanced and more fundamentalist collection of Christian-majority countries which hold a fair amount of resentment for imperialist “Islamistan.” Another notable country is Japan, which is mostly unchanged by the presence of “Islamistan” up to a certain point.
Other changes to the timeline include Arabic being the most common language in the world (rather than English); the industrial revolution (perhaps) being spearheaded by Mongol China and at a different time than IOTL; a minority Christian group called the White Lions exists; depending on previous changes and historical precedent, the Native Americans may be better-off than IOTL; astronomical and geographical features named after Greco-Roman figures and deities are instead named after primarily Egyptian but also Semitic and Indo-Aryan gods (such as Mars being known as Sekhmet instead); and the headquarters of TTL's version of the United Nations being located in Jerusalem.