I mean, the easiest option IMO would be to have some way of introducing horses to the continent earlier - we saw how IOTL that pigs and horses both exploded beyond their initial populations and "habitats".
So I'd suggest that a potential (relatively late, admittedly) PoD could be a Vinland-Lives scenario, effectively a larger group of Scandinavians and Northern Europeans make their way over for some reason - perhaps we see it happen earlier - it is suggested that Gunnbjörn Ulfsson found Greenland approx a century earlier, so we could have that bring our timeline forward and give ourselves a potential century more to not just develop some homesteads in Newfoundland, but further south and even around the St.Lawrence. Bringing over cattle, pigs, and horses and you've enough of a timeline that horses can spread to the Great Lakes, and onto the Mississippi, but because only the North is aware of the region, it isn't that wealthy, and can still be cut off or be isolated over time to vaguely isolate things.
Now, it should be noted that there were already significant groups, like Cahokia in the region, but if we introduce the horse about 4-5 centuries before Europe arrives, that's a substantial period for horses to spread, be adopted, and even help transform societies - like allowing easier upriver barge traffic.
Though an early introduction of horses would likely mean a collapse in the Bison herds. By 1840 Indian hunting of bison was already unstainable