PoD: Prevent Alexander the Great's vast conquests. An alt-king of Macedon limits his conquests to Anatolia and lives to a ripe old age allowing for the creation of a united kingdom of Greece and Anatolia. This hypothetical kingdom acts as a check to Roman power in the east.
And if you can somehow avoid the destruction of Carthage that would give you a check to Roman power in the West. Rome can still become a respectable power in its own right, but it would likely never conquer the whole Med.