Also check out geographic counterfactuals in fiction
I'm nerdy enough that I keep a list of possible branchpoints in history where it would be interesting to explore counterfactuals, and which (to my knowledge) no one has. Originally I was going to write them as short stories, but since short stories take longer to write than blog posts and in alternative history the characters and dialogue are often afterthoughts, so this would seem a better venue. Starting with this one I'm going to go in chronological order of the branchpoints.
First: there are neanderthal alternative histories. There's a Harry Turtledove story about a present where Neanderthals have their own nation-state in the dry Mediterranean salt beds, as well as (excellent) Robert Sawyer novels about a Neanderthal physicist from a parallel universe where sapiens died out.
But in these stories the neanderthals were of equal intelligence; the point of alternative history, as a subgenre of speculative fiction, is to bring setting into play as a variable among the elements of literature to ask questions that would have otherwise been senseless or impossible (discussed here.) In this case, that question is what happens if there is an intelligence gap? In fact there were erectus-descended hominids still on Flores until 10,000 years ago or so, and presumably the arrival of humans didn't help them very much. So the first problem with such an alternative history is it's hard to imagine a coherent sequence of events that would have allowed erectus, if they're less intelligent than us, to have survived to the present unless they're in complete isolation - and the converse is true as well. Harry Harrison explored the same idea in East of Eden, where humans isolated in North America meet the highly advanced descendants of dinosaurs, previously unknown to them, as they push out from the continent-spanning city in their home in Africa. Once contact is made it's hard to imagine a good outcome for both species. Might this not be truer the more biologically similar they are? After all, reptiles and mammals aren't competing for the same resources.
So, perhaps imagine an alternative history where eighteenth century English sailors talk to Chinese fisherman off the coast of Australia, and take the Chinese myths of small hairy men they met on the shores of a vast southern land to be just old wives' tales - and then find they're true. If the representatives of King George had found Australia carpeted with the glittering crystalline technology of an erectus nation-state, all of whose citizens spend the day in quiet inward-looking meditation (which is why they haven't colonized the world), might they not have been well-advised to quietly turn around and sail back out to sea, hoping not to have been noticed?
What if that spear were a ray-gun?
Conversely, let's assume that today, there is a mistake that's been buried in satellite code since the first launches that has resulted in a certain latitude-longitude blind spot in the south Atlantic, where there turns out to be an island harboring barely-tool-using erectus. With our modern sensibilities, would it be moral to make contact with these "people"? Could we make contact with them, if they can't fully understand language? Meaningful and fair trade would seem to be out of the question. They could barely be assimilated into the global economy as manual laborers, and their island would be quickly swindled out from under them by the tall pretty people who came to visit them. What could be a possible positive outcome for them? In 2100, will humans again accept serfdom as a moral alternative for these not-chimps-but-not-people? Or should the U.N. keep them isolated as a nature preserve, defending the island from smugglers and poachers, and darting the curious ones that swim out to the observation boats, to be replaced back in their brush-nests before they see anything?