Civilization 7's leaders represent a major break with the series' traditions, in that they're completely separated from the civilization you choose. You can mix and match with total freedom. Want Ben Franklin to lead the Romans? Sure. Pharaoh Hatshepsut at the head of the Mayans? You bet. However, while you'll choose a new civilization for each of the three acts, your leader will remain the same throughout. So picking one is the most consequential playstyle choice you'll make before starting that first turn.