What-if instead of a defeat at the Battle of Salmis, Xerxes either wins, or doesn't bother falling into the trap.
This battle was the turning point in the Greco-Persian Wars. Had Xerxes not been defeated who knows how history would have changed; so dear denizens of HWif, how would things be different with this point of departure? Would Xerxes roll up the Greek states, make them all vassals, end Western Civilization or create a newer freer world. What-if?