You could add the Americas in some fashion as a part of the game, maybe an expansion to the map of the late era or maybe as its own campaign, you already introduced what is basically the crusades campaign from medieval 2, you could add many playable factions like the aztecs, mayans, tlaxcalans, apacheans, chichimecs, tarascans, new factions like the powahatan confederacy, the cherokee nations, the iroquoi confederacy, the huron, the pueblo, the plains, and even somehow the Incans, you could even have part of newfoundland canada as a part of the early campaign since in actuality the vikings colonized that region in the early middle ages.