What if the second child of Olympias with Philip II had been a son, a healthy boy, instead of a daughter?
Philip having not the need to secure his succession by having a new son, he doesn't marry Cleopatra Eurydice who becomes instead the wife of Alexander. She becomes pregnant and gives to Alexander a son before he begins his conquest of the Persian empire.
Of course, after the assassination of Philip, Alexander had his brother executed.
Alexander dies at Babylon, in june 323 BC as IOTL, while his son is 12 or 13 years old.
What could be the consequences in the power struggle following the death of Alexander III the Great?