In this ATL, the Roman Empire was reformed instead of being divided into east and west, preventing its decline. The Roman armies managed to defeat Germanic and Celtic forces, capturing the rest of Northern Europe, before moving on to capture the more eastern lands, where they would clash with 'barbarian' forces from Mongol and Turkic lands (something that would continue for decades). With the capture of most of Europe, Rome turned its focus towards the Near East and Africa, which they eventually captured as well, acquiring large amounts of slaves.
Meanwhile, the Mongol Empire had managed to survive a series of internal disputes threatening to divide the nation, and reaffirmed its position as the most powerful nation in Asia. Mongolia managed to get other Mongol and Turkic states to join its empire, paving the way for a takeover of the rest of Siberia and other lands, before capturing India with the help of the Mongol-descended Mughals. Mongolia struggled, however, to take over the insular lands of Japan and Southeast, but once a decent navy was created, these areas were captured as well. Mongolia then discovered Australia, eventually capturing it as well.
At this point, Rome and Mongolia, each with several client states, were by far the world's leading countries, clashing periodically in areas where they bordered one another. This delayed the discovery of the America's significantly and prevented the disastrous disease outbreaks, allowing the Inca and Mexica (Aztec) Empires to continue existing, with the former, however, experiencing a civil war. Both empires expanded, assimilating other cultures into their own, and eventually, they had split the America's between them, with the Incas controlling South America (called Inca) and the Mexicas controlling North America (called Mexica). Once the continents were finally discovered by the Old World, the strength of the Amerindian empires prevented the colonization of vast areas. Nevertheless, the Mongols managed to take over the Pacific Northwest, while Rome captured lands in the northeast. These foreign empires were perceived as a major threat by both the Incas and the Mexicas, leading to a military alliance between the two and a common culture. Antarctica was later colonized by the Incas.