I pretty sure this idea may have been proposed, but what if the Megafauna Extinction at the end of the Ice Age didn't occur? This means North America would be populated by various mammoth, tapir, rhino, and big cat species. And more importantly for natives that came to America: peccaries, horses and camels.
How would history be changed? Would we see earlier contact between the continents? Would the exchange between the Old World & the New World be less one sided? What would colonization look like in this scenario?