AD 500 sounds about right to me too.
I would run Dark Ages from roughly AD450 to AD1100. Fall of Rome to the Norman Conquest and 1st Crusade.
Medieval roughly AD1100 to 1500. Crusades, Reconquista, up through the HYW and French Wars of Religion. When gunpowder starts to become prominent on the battlefield time to break.
Is each of these going to be its own game? Or are they going to be supplements to MA:Ancients?
Is there thought of doing Colonials, 18th century, ECW/TYW, Modern, Sci-fi etc?