@derp Some interesting thoughts.
I am gonna blether 'cause I think it's interesting and think you might also.
There's this theory that the reason we experience the "Uncanny Valley" effect is that 50k years ago and such, we did have contact with other Homo-genus hominins, and most of the time they were dangerous af.
I am much amused by the one about islanders in Oceania having legends of scary little people who would eat you, but who also kidnapped older children, who would easily escape because the scary little people were not clever. The scary little people would try to force them, the kidnapped children, to teach them, the scary little people, how to cook, but were unable to learn the skill. This whole thing is probably made up by cryptid-fans, who say the scary little people were Homo floresiensis.
It sometimes sends people who aren't into taxonomy 'round the twist, because it's contrary with what they teach in basic schoolkid bio, but species is a difficult concept - 'broadest group that can still create fertile offspring' doesn't cut it. Sometimes hybrids that are normally not fertile are fertile, sometimes fertile hybrids are possible but never happen in nature, often fertile hybrids are possible and happen regularly.
"Species complex" is a term for a group of species where the edges are unclear. Canids of the genus Canis are a good example. Domestic dogs, wolves, coyotes, golden jackals, Ethiopian wolves, Red wolves, African golden wolves, and dingos are all (usually) classed as different species (sometimes with dogs, wolves and dingoes lumped) but they can all interbreed and create fertile offspring. They don't seem to have much problem communicating with one another, though there are some pretty marked species-specific behavioral differences.
I think the classic D&D "races" make for interesting speculative fiction about what it might be like if humans were part of a species complex. I kinda like to include this notion that one of the morphological differences is in vocal apparatus -- the "goodly races" have type 1, and the "goblinoid races" have another, it's physically impossible for a human to speak orcish or goblin properly and vice versa, (almost) only orcs or half-orcs can do both, much stupidity has resulted.
Also, endless details of culture and cross-cultural relationships. Excessive world-building vs racism, I guess.