We should definitely not take periodization too seriously, though it's fun to discuss. If I had to divide all of world history, I'd first look for a common theme, and the one that jumps out at me is increasing globalization. This isn't an original argument, but if we think of world history in terms of growing interconnectedness, distinct eras emerge at least on the broadest scale: The largely isolated "Cradles of Civilization" in Antiquity e.g. the Fertile Crescent, the Indus River Valley, the North Andes. The relatively discreet historical theaters of the classical period e.g. the Mediterranean, the Sinosphere. The mostly separate "worlds" of the Middle Ages; Afro-Eurasia and the Americas being the big ones. The globalized world of the modern era. In this conception, the global Middle Ages would begin around the time the Silk Road and, more importantly, the Indian Ocean trade routes began in the early centuries AD.