An idea that's been on my mind for a little while now is having demihumans not be separate races in their own right, but humans who, by birth or by other means, are something different. In other words, two humans can give birth to an orc or elf if certain metaphysical elements line up. It's difficult to verbalize without sounding pseudo-scientific. The last thing I want to do is make something that I understand as patently magical to be communicated in a scientific way.
The best way I can explain it is to give an example. The names I'm using will be generic since I haven't come up with proper names yet.
[example]
Every few generations, a child that is half mortal and half spirit is born into the Tribe. Sometimes a spirit comes to a woman while she is alone or while she dreams and gives her a child. Sometimes a man has a strange encounter with a beautiful woman during his vision quest and almost a year later a child is left in the middle of the village. Though the Tribe venerates the spirits and views all children as blessings, they understand that not all spirits bear mankind good will. They can be malevolent and capricious, using the Tribe's greatest blessing, their children, to do them harm to to amuse themselves. For this reason, the birth of a half-spirit child is treated with a mixture of wonder and fear. Wonder for the great power they inherit from their spirit parents. Fear for the fact that at times this power can be greater than their wisdom or their kindness.
Since witnesses are hard to come by in matings between humans and spirits, the Tribe relies on other means to determine whether a child is half-spirit or not. Dreams are the most important part of determining what the child's parentage is. The most telling evidence comes once the child is born. Although spirits in their natural forms do not have bodies, they leave their mark upon their half-mortal offspring. In form they look human enough, but there are a few very odd details. Many are born pale-skinned, sometimes even chalk white. Sometimes, though, they have skin as gray as a storm cloud, green as summer leaves, or blue as the daytime sky. Spirit children are also known to have hair the same color and texture of cornsilk or sheep's wool, but any color and texture is possible. Spirit children tend to have eyes of an unusual shape, size, or color. Blue, green, or gray eyes, are common (if anything about these children is common) but some are known to have red, violent, or even amber eyes. If a child is an albino, that is a clear indicator of spirit parentage. A child that grows hairy when he starts to come of age probably has a spirit parent. Fangs and claws are not unknown; they can even have pelts, antlers, or hooves!
[/example]
From this little snippet alone, you can conceivably have all the standard D&D races, plus more than a few extras. However, as individuals none of these creatures are a particular "type." There aren't any stereotypical behaviors or attitudes that come with being one of the spirit children. Each character's individual experiences influences their personalities and outlooks more than their race, so to speak. Strangely enough, you don't even need to represent this mechanically. A human who looks different from the norm can be deemed one of the "spirit children" while having none of the supposed powers of their alleged spirit parent. I had a point about this a moment ago, but I forgot it.