Old World at the end of the Medieval Period, loosely inspired by a Crusader Kings game plus another idea.
What if China successfully beat the steppe nomads and prevented the formation of the Mongol Empire, replacing it as the dominant world power of the 13th century.
The Song Dynasty was in a downward spiral, losing ground to the up and coming Jin in the north. The Jin gave rise to a new dynasty that was able to unify China and also acquire horse warfare expertise. Due to the increased raiding activity of the yet-unified Mongols, China determined to secure the northern border using several strategies, at first using one similar to Emperor Wu's of the 1st century in his successful pacification of the Xiongnu, which was the model of the new Western Expeditions.
The idea was to control centers of commerce and supply, blocking any wealth-generating areas from providing tribute to the nomads. The nomads, cut off from external supply would be weak enough to face down with newly formed Chinese mobile cavalry-based armies. And then, the pacified steppe lands would be colonized by loyal populations bringing in cattle and sheep to create a new pasture economic model that would feature fewer horses, and thus less military potential.
The Chinese border expansion policy took them into the Tarim Basin, and from there into Central Asia, where China came to dominate most of the Silk Road.
However, there were issues finding people willing to emigrate from the Chinese mainland into non-agricultural lifestyle of cattle herding, so China began a policy of accepting foreign settlers from Europe and Persia which were engaged in numerous conflicts along religious lines and thus generated plenty of refugees and captives.
Meanwhile outside of China, Egypt has given rise to a new Jewish faith, only militantly evangelical like Christianity and Islam, following their founder prophet and Messiah who took the name "Kurosh" (referring to Cyrus the Great who biblically liberated the Jews from Babylon). This new Misraic Empire overthrew the Arabo-Muslim rulers of the region, liberating Africa and Iberia, also re-establishing the Kingdom of Israel. In the West, many Muslims were forced to convert, while in the East, Muslims rallied around Persia. In line with the religious shift, there is also a re-alignment of linguistic priority, as Egypt and Africa officiate a resumption of pre-Arabic Coptic, Berber, Andalusian, and Hebrew language in the administration of their respective regions.
In Egypt proper, new construction projects overhaul the urban design of the Nile settlements and improvements to agriculture, trade, and manufacturing result in an economic boom, making Egypt the most economically prosperous place on Earth.
Ironically, defeating their Arab rivals allowed the Romans and Persians to recover and bounce back to new heights.
In Europe, the Holy Roman Empire never managed to finish expanding into Germany, remaining a primarily Franco-Italian production. As a result, Polabia became a powerful catholic kingdom east of the Elbe.
The Hungarians never migrated, so Pannonia is Wallachian/Romanian. Bulgarians and other Slavs have become very important parts of the Roman Empire, though there is still a separate independent kingdom of Bulgars which resists Hellenization.