9. The Islamic Golden Age
Technically, the series already has a game set in this time period: the very first one. That said, it did very little in the way of exploring the Golden Age.
You see, the Islamic Golden Age -- roughly the 8th through 13th centuries -- saw the Islamic world eagerly embracing science, technology, art, inclusivity, medicine, and knowledge, while Europe was intellectually neutering itself with the Dark Ages.
Baghdad, the capital of the Abbasid caliphate (it was also the world's largest city at the time), became home to countless libraries and universities, as well as the Islamic world's first astronomical observatory.
Also established in Baghdad was the House of Wisdom (alternatively known as the Grand Library of Baghdad), which saw philosophers, scientists, and scholars -- be they Muslim, Christian, or Jewish -- getting together to share their ideas, at which point they were translated into Arabic.
All that to say, it was one hell of a city.
The game could see the end of the Golden Age, as attacks and invasions by the Mongols threatened to destroy the Abbasid caliphate. It could even feature the 1258 Siege of Baghdad, which saw the House of Wisdom destroyed by Hulagu Khan's forces.