[EDIT] Civilization list as of 2020-07-17:
Millennium A.D. covers the AD 500–1000 timeframe, possible civilizations include:
Anglo-Saxons (England, 410–1066)
Carolingian Dynasty (Western Europe, 751–888)
Norse (Northern Europe, 793–1066 ‘Viking Age’)
Kievan Rus' [Ukraine, Belarus, Russia, 879–1240)
Avars (Central Europe, 567–822)
Great Moravia (Central Europe, 833–907)
First Bulgarian Empire (Balkans, 681–1018)
Khazars (Eurasia, 650–969)
Byzantines (Eastern Roman Empire, 395–1453), especially the Macedonian dynasty (867–1056)
Arabs (Near East and North Africa):
Rashidun Caliphate (632–661)
Umayyad Caliphate (661–750)
Abbasid Caliphate (750–1258)
Tibetan Empire (Greater Tibet, 618–842)
Tang Dynasty (China, 618–907)
Balhae (Manchuria, 698–926)
Later Silla (Korea, 668–935)
Classical Japan:
Asuka period (538–710)
Nara period (710–794)
Heian period (794–1185)
Optional:
various Germanic kingdoms during the Migration Period:
Vandals (Western Mediterranean, 435–534)
Visigoths (Southern France and Spain, 418–721)
Ostrogoths (Italy, 493–553)
Lombards (Italy, 568–721)
Burgundians (Switzerland and Southeastern France, 411–534)
Old Saxons (Northern Germany, 531–804)
Bavarians (Austria and surroundings, 555–976)
Aksumites (Eritrea, 2nd C – 10th C)
Nubians (Sudan):
Nobatia (5th C – 7th C)
Makuria (5th C – 15th C)
Alodia (6th C – 15th C)
[EDIT] Civilization list as of 2020-07-17:
Millennium A.D. covers the AD 500–1000 timeframe, possible civilizations include:
Optional: