America without plagues

PLEASE COMMENT

This picture is a conversation piece I'm posting, based on a conversation I had with .
MOAI is working on a story project about an alternate world in which the war between the Native Americans and European settlers didn't end quite so badly for the natives. It's an alternate history that jives with a story project _I'm_ working on, and I thought I would share my thoughts with you all, and see if anyone out there can add my and MOAI's worlds.

So, here is the scenario. When the expeditionary forces of Spain and Portugal make their landings on the Americas, they do not find decaying empires thrown into chaos by plagues. In this history, the Americans are immune to European diseases (perhaps because, as in MOAI's world, they have their own domesticated animals to transmit diseases to them and build up resistance in the population). If 1491 is to be believed, then, America at the time of Columbus was teeming with people, some of whom lived at population densities that rivaled Asia (and far surpassed anything in Europe). What happened next?

Well, I actually wrote a whole world history just now, but let's tone down the nerdiness, shall we. Let's just look at this blank map.
Okay, here we have Western and Midwestern North America. You can see a big centralized state around Mexico, which is pushing up into the NA southwest. Many small principalities around the mouth of the Mississippi, which forms the core of a cultral sphere of influence that extends all the way up through the Great Plains to Montana and Minnesota. This area is something like Spain and something like China, a collection of usually independent states that periodically form a federation or centrally controlled empire. On the West coast, there are a group of wealthy states, perhaps colonies of Asian powers (Russia, China, and Japan?) They don't have much to do with the Plains peoples. To the north, the Plains Federation fades into horse nomads, who occasionally invade southward.
The Mississippi forms a natural barrier between the Plains Federation and the eastern states. They aren't on the map,
but we can expect a collection of allied principalities around the Great Lakes, and another, larger, richer, and more powerful, around Florida and the Gulf of Mexico. The Appalachians are rich in coal, and might be the site of an industrialized nation. The banks of the Mississippi support rich farmland. The area around Boston and New York probably support trading centers, possibly the centers of countries. We can expect at least some of these countries to have strong European influence, in the same way modern countries in Africa, Asia, and the Middle East to retain influence from their colonial conquerors. The people who run these countries probably speak English and French, and possibly Spanish and Dutch (like their counterparts on the west coast speak Russian), but let's assume that, like the old colonies in our old world, these colonies have regained independence.

Whether the Americans' relationship with the Old World powers are cooperative or competitive depends on a lot of things. How easy was it for them to gain independence? What atrocities have been committed on both sides? How did Europe come out of World War Two without America to beat back Japan and hold a hammer over the USSR? I think I will leave that up to you all.