One of the TL-ideas I toy with but never write is inventing a Native empire POD's around 1000AD in the "East-North-Central", probably on the Ohio river. I would try not to model it on the Roman Empire as closely as Turtledove's CSA is OTL's Germany, but the general idea is:
- have a coherent, well-managed empire with Urban centres, a good degree of technological, economical, scientific [medical!] and military potential and a population of (even after the European diseases rush through the continent) several million.
-it needs to have a good deal of cohesion. Of course, most of the realm would have been conquered, but like Rome it would have an ability to integrate those regions in the following decades
-it is an "Inland Empire", its naval abilities restricted to rivers (which are its main lifelines) i.e. when the Europeans arrive on the seaboards, these regions will not belong to this empire. But its existence and importance would be known throughout North America.
I imagine a steady, "organic" growth of the empire and its culture until, when in the early 18th century, the European "Frontier" is at more and more places a "Border".
At this point of time, this Empire might basicallz reach from the Tennessee to the Great Lakes, from the Appalachians to the Missouri.
-they are technologically inferior to the Europeans to a degree, but adaptable enough that they would never be more than a step behind, a gap which would even close as time progresses.