Humans are already almost as diverse as canids (wolves, dogs, coyotes) as a whole. Until the Bantu arrived in Southern Africa (I think less than 300 years ago, definitely les than 500) the San has been reproductively isolated for at least 200,000 years. You don’t need to wonder what that world would be like, you live in it. All non Sub Saharan Africans are ~2% Neanderthal. Papuans are 5% Denisovan, from probably three different sub populations, likely as different from each other as the major continental ancestry groups of today. Tibetans have a high altitude adaptation from Denisovans at more or less ~100% fixation that is almost absent among the Han Chinese they otherwise closely resemble.

We fought with, killed and mated with the other kinds of human. They may not all still be here but there’s a lot of them in us. Humanity mostly comes from Africa around 200,000 years ago but there’s a lot of deep population structure that’s much older than that.