The megafauna can be saved by having people head over to the Americas *earlier*. The Old World species (mostly) survived because their first exposure to humans wasn't Neolithic hunters who could bltiz through two continents worth of animals not used to regarding little pipsqueak monkeys as a serious threat. How you'd get hominids in the Americas earlier is difficult. I'll rank three in declining order of likeliness: (1) Neanderthals hunting something on a glacier end up taking a cruise when the glacier calves off into an iceberg. It appears from current evidence that Neanderthals hunted in mixed-sex groups, so this puts a breeding population with a food source (dead mastadon or whatever) into the ocean. They drift across and end up stranded in the Americas someplace. (2) Hominids (of any stripe) along the West African coast take an unscheduled trip on a mangrove raft. Less likely to have a whole tribe taken for a ride. OTL, this is how monkeys reached South America. They'd end up somewhere around Surinam. (3) Parallel evolution of bipedal hominids from some neatropical primates. In any case, they spread out and adapt to the local conditions. In all three cases they should end up in pleasant environments, from their POV - the Neanderthals should end up in parts of North America like the Europe they left, the Africans would end up in another tropical rainforest environment, and in the third case this would be home from the start. Anyways, if you have *that* setup, it's probably even more interesting than just having megafauna, as you'd have another species of humanity with varying degrees of relationship to h. sapiens.