Yes, I'm already thinking about what new factions can be added in later on.
1- An Ethiopian-inspired faction, taking up the aesthetics and feel of 4-8th Century Aksum and Himyar, maybe even with a nudge of Egyptian flair (very appropriate for the Nubians too). Let's call it Kush.
2- A Persian or Indian-inspired faction heavy on foot archers and heavy infantry. Let's call it the Padishash (Rhaegea is said to come from "the realm of the Padishah" in the game files).
3- A "holy order" style of faction similar to the Teutonic Knights, the Hospitallers or the Templars, maybe set in the empty land down under the Nahasa desert, at the coast.
4- An expeditionary Chinese or Japanese force, based on the Treasure Fleet of Zheng He.
5- Early Medieval Iberia? A man can dream.