Jeremy Ackerman-Yost
Explorer
There are already human sub-races in the book since PHB1. Shifters, Shades, Vryloka, Tieflings, etc... you get the picture. They all are half-humans or have human origins.
Sure, but the semantic difference is important here. They aren't "human subraces" the way Gold and Shield Dwarves are "Dwarven subraces."
The result of humans mating with random stuff is fine. It's sort of distant from reality while simultaneously fitting human behavior just fine. Subdividing plain old humans would cause a bit of backlash. It's a touchy sort of subject, with good historical reason.