To answer the OP:
I've always wanted to do something on the Mississippians, where they can take their place next to Mesoamerica and the Andes as a great center of civilisation. It probably would involve the Eastern Agricultural Complex not being so displaced and being more fully evolved, although they'd still import Mesoamerican crops. I can imagine sites throughout the Mississippi, Ohio, Tennessee, etc. valley being full of incredible mounds like Cahokia is, in a way that makes Cahokia look nothing special. Even smaller Mayan (say) sites seem full of incredible structures in a way only the larger Mississippian sites are. I'd love a way to get something that complex in North America. Maybe we get Hernando de Soto's name as famous as Cortes and Pizarro as a great conquerer. Or maybe we get his name as a man who tried to imitate them, but utterly failed.
One thing that bothers me is language/cultural-related problems, since archaeologists obviously can't tell what language these people spoke, and the people encountered by the 17th century French in the same region seem to be very culturally different. It seems to be Siouan languages, and without a doubt Dhegihan Siouan languages were involved, but what about the other languages in the area like the Tunica, Natchez, Yuchi, and other tongues? And the cultures, of course. I guess I'd also have to go with the Lands of Red and Gold method of using OTL names for important places, since indigenous American languages are a headache to even try and figure out (although from what I've seen, Quapaw and other modern Dhegihan Siouan languages don't seem so bad compared to Tunica, even if I'm obviously making something up, but we don't know the OTL name to begin with)
Maybe one day this'll get written.