I wrote a quick page of flash fiction sketching out a world where Chinese ships reached California a couple hundred years before the Conquistadors reached South America, tl;dr the Chinese empire can’t exert power on the continent the way the European empires could, indigenous cultures adopt some Chinese technologies and state organization principles, and the Spanish and English arrive on schedule but find their ability to conquer stymied by Aztecs with arquebuses and cannons.

My second choice is that a major slave rebellion is successful in Ancient Rome, resulting in a class-based overthrow of the previous state apparatus. The former slaves reform the existing Patron-Client networks to eliminate the patrons all together, resulting in a society economically organized around circles of associated producers and craftspeople who are of equal social rank and work together while sharing a commons. Eventually as the Roman state breaks down from the inevitability of its size and outside pressures, the resulting “fuedal” system is not one dominated by noble lords and their systems of alliances, but a complex web of cities and townships which associate freely with one another, slowly forming into complex realms with distributed, instead of centralized, leadership structures.