I've been a bit too aggressive in this thread so far, and I don't want to kill it off by beating up on every idea that other people bring up, so I'll back off for a bit.
I've been thinking a lot about megafauna and evolution timeline concepts, and my thinking has focused on a couple of main angles:
First of all, we have to accept that, whatever killed off the megafauna was very quick and very dramatic. So, unless a timeline proposes some major, dramatic changes to history, with rampant butterflies, one probably shouldn't be expecting the megafauna to survive intact. In order to make the timeline manageable and realistic, I would prefer to pick just a couple species to survive and try to contain the butterflies to some extent.
Unless, of course, you're interested in taking on all the butterflies and effectively rewriting history with a clean slate. The uncertainty about what caused the megafauna to go extinct gives us considerable wiggle room in determining which species might survive the onslaught. We don't really have to explain why X or Y survived, because we really don't know why it didn't survive in OTL.
Likewise, the paucity of historical records about Paleo-Indians gives us precious few insights into potential points of divergence, and potential alternate courses of history. For example, what if Paleo-Indians had never hunted mammoths? Would anybody raise their eyebrows over this? The Olmecs apparently had invented wheels, but they only used them on toys, and apparently never made chariots or other vehicles. Is that any stranger than not hunting mammoths?
I do not, however, like to abuse the audience's willing suspension of disbelief. There is a tendency to go over the top with megafauna timelines. Once you've gone a certain distance from OTL with megafauna, you might as well just pull stuff out of your rectum. We can just pretend that evolution or artificial selection will let us get away with anything. It's very difficult to give feedback on that, especially dissenting feedback, and I think this is why most megafauna threads don't get very far. (There's also the fact of biology nazis like Sven wanting to beat down every new idea someone comes up with).
There are good opportunities for collaboration though. For instance, we could pick an animal, and decide, after much debate, what characteristics it would most likely have, what characteristics it could most likely be bred to have, and then start in on how these characteristics would make society develop differently from OTL. For instance, maybe there's a surviving North American camel that is domesticable. How would a New World camel society develop differently from the Old World horse society or cattle society?
If there's interest, maybe I'll start a new thread for that, so as to avoid stepping on Argo's toes.