Also the Dakotas...more important than Kansas, oil-wise. However, the majority of the oil in Texas in in the Gulf-East Texas region or in the Permian Basin--part of the latter is on the Great Plains, but part isn't.
Honestly, doing so wouldn't be any different than almost any previous national park set up by the United States--even leaving aside the indigenous population, many of them were founded in areas that saw at least transient use by neighboring populations, leading to quite a bit of conflict as a result.
Pretty sure that was the idea, yeah.
Realistically, though, a smaller-scale version of this is likely to develop in the future as people move off the plains and leave large regions effectively abandoned (especially if Ogallala goes and you can't irrigate most of the plains any longer). Major cities like Omaha or Kansas City will remain, of course, but other areas will be functionally depopulated. This would create a de facto version of this. Conservation organizations and maybe the federal government would also in that case have more ability to buy up plains land and set it aside for conservation, which might lead to the creation of formal parks in some areas.
That's more or less what the Poppers are talking about now, anyway. Continued decline in rural economics in part of the Great Plains (not the whole thing) leading to large parts essentially returning to nature. They compare it to the more remote parts of New England, where attempts at cultivation in the 18th and 19th century were abandoned and led to those areas returning to a natural forest environment.
