Personally, I'd love Makuria (Orthodox Christian Nubia).

I think it would be far better done if they had different start dates (E.g. 1080, 1205, 1321, 1450) and you get to take the Aztecs as playable (with very weak troops compared to Europeans and a reform option to allow better troops if the in the last.

Also, I'd like to see Europeans competing to reach the new world, so the date of discovery (and strength of expedition) varied with the stability of Europe and thus the Aztecs would have more or less time to prepare.

Regardless, adding Nubia and giving a fluid Egyptian reform (Fatimid-Ayyubid, based on religion - Salah al-Din is a powerful Shi'a governor, worse infantry and archers, better and more numerous cavalry) with civil war included would be nice.