North America-incredibly unlikely(maybe if a lost colony of horses survived and are discovered)-maaybe say a plague strikes the Europeans and Ottomans and wipes them out horses still enter the Americas and you have empires from there
South America- Wanked Inca Empire unites with Amazonian civilization-very very difficult
Europe-OTL has shown more than a few attempts at this and it hasn't worked so far-a few possibilities, wanked out Roman Empire, Alexander conquers everything from the Atlantic to Peking, united Hapsburg Empire(they stomp all their rivals and get everything but Britain), maybe muslim Europe(double blow if Arabs win at Tours and take Constantinople), Mongol Europe, Russia in the 19th century, France in the 19th century
Asia-maybe mega Mongol empire(same for Europe) but very implausible, wanked out Chinese empire, uber caliphate, uber Alexandrian empire
Africa-the british keep the other Europeans out, wanked Islam, wanked(beyond the stretch of imagination) roman or Alexandrian empire
Australia-maybe an aborigine tribe develops agriculture and conquers or vassalizes the rest of the continent.