The best start date IMHO would be 367. In 367, Perdiccas III had taken the throne after killing his own brother King Ptolemy. Perdicass III's other brother was Philip II father of Alexander the Great (unfortunately he cannot be accurately included from the start in this setup as he was born 3 years after his uncles death).
The Greek city states were weakenned by this point but by no means anaemic. Macedon was by no means dominant at this point, there had been 5 kings and a civil war in the last 20 years, but was emerging.
Persia was still the dominant force in the world at this point, despite many recent setbacks they would soon reclaim the Greek territories in Asia after this date.
Lots of battles to be fought at this point as the old empires declined and the new ones rise.
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