I was thinking this:
A unit line for the Northern Natives, shared by the Haida, Inuit, Cherokee, Sioux, Navaho and Iroquois. instead of using the European-ish Firaxis Spearman, Pikeman etc.
A unit line for the Southern Natives, shared by the Azteks, Mayans, Maphuche, Incans, Tupi, Jes, Caribs and Arawaks. This would start with the Tupi units that are not also UU's for any of the civs already, and later progress to units the Incans, Azteks and Mayans built normally(UU's for those three tribes), or more generic units for the rest of the bunch. More Latin American like units would come later
As for the current American nations, they could progress like the Natives, only they'd get European stuff starting in the Colonial Era.
When I'm done with playing Baldur's Gate, I'll show you what I mean(I'm going to work on the ACM)