[quote=@Nerevarine]
The Idea of the West being mostly uninhabited make sense, plus it could lead to some interesting demographic changes and conflicts in history. We could also have unclaimed land be mostly tribal or other disorganized groups, or things like City States. 

For now, all unclaimed land is available


[/quote]

To be honest with a collapse of modern industrial infrastructure to pipe in water for irrigation, most of the American west is empty land anyways and farming is minimal. This is also pretty much ignoring the over-all fact that a vast majority of the acerage in those areas is currently federal land (military functions, ie the property around missile silos and relevant bases, national parks, etc.) and what private land there is would all be cattle ranching.

Chances are that any societies living on the plains in the north there east of the rockies would likely take on a nomadic life-style in the same way as the Sioux Nation (Lakota, Santee, and Yankton-Yanktonai). Or in a old-world comparison: the steppe Turks.

Given long-enough time after a collapse in modern infrastructure to hold the Buffalo at bay and a over-all drop in human populations there might even be a resurgence in the Buffalo populations; more given the current trends to start ranching buffalo like we do cattle in Idaho and Wyoming.