I've been experimenting with giving the various races unique powers that individuals can occasionally obtain (as well as generic powers any person of any race can acquire.)
There's new races arising during worldgen, experiments like the necromancers do. Not always by evil needs to conquer, but sometimes experiments such as attempting to understand the nature of the soul or creation.
Gods altering existing races, again.
Iirc some stuff Toady pulled from the myth generator prototype included races that were angels or created through magic in some sense, being devoted to particular deities or forces. Maybe elementals too, like rock or gem people creating societies. Will ice people survive outside the frozen tundra? Maybe they're magically protected from melting up to a certain point. There's a note in the raws about blizzard men should be made out of ice/snow. Nobody will enjoy having magma men around if they don't stop setting everything on fire.
I also read on some wikipedia article about Philippine folklore, something about an extremely wealthy and powerful tribe of people who hid themselves away through magical means, and could abduct other people and force them to eat black rice, indoctrinating them into the tribe. I also recall some Hawaiian legends about a race of "little people" who built some things on the islands before disappearing or being killed off. Maybe lost tribes (or hamlets/hillocks as the case may be) due to magic are possible? Imagine a plane that consists exclusively of a single town inhabited by a reclusive group of magic users who don't want visitors.
Oh, and who can forget all the dragon-offspring in D&D. Half-dragons everywhere. Magical creatures coming into town and leaving them stuck with a bunch of hybrid children or children with special abilities. The escapades of Zeus.
Or just as silly, magically intelligent animals. Not like humanoid animal people, just intelligent animals created by gods or forces or regions. There is a knowledge sphere, what if regions beholden to it often produce magically intelligent plants and animals? Or animal-people coming from not savage regions, but regions with spheres like knowledge, or deformity; people going in and coming out changed into part-animal/animal-person/centaur-like forms, or entirely turned into talking animals. How would a society of intelligent kangaroos survive? Probably not well, I've heard kangaroo paws arent particularly dexterous. Better than the snakes will fare, though.