*I really like warforged. They need a place in this world. I like the idea that they are fairly new, (or maybe just recently discovered?) but that's as much as I've got for them.
(Inspired by the clay Chinese warriors of Qin, Cybermen, Daleks, and countless others) The Warforged in my campaign work in progress were designed by dwarves (playing the role of neutral weaponsmiths) to be the recepticles for the spirits of psionically active humanoids using them as war machines. At some point before delivery could happen, the Dwarves were all killed, but a few took advantage of the Warforged and transferred their consciousnesses.
The Inheritors, as they now call themselves, are thus an amalgam of Warforged physical stats and Dwarven cultural stats, but with Darkvision and Wild Talent added. (I'm also considering having newer Inheritors having a more Dwarven appearance..._
*I want to include a group of people like the Tuatha Dé Danann. I think that either a) they are a long lost nearly fallen race, just shadows of their former selves. (which means that they are a PC race. ie. Elves), or I like the idea that the Tuatha De Danann are a group of something like Old Elvish demigods, and that they rule over all the fey and elf type creatures. I want there to be seelie and unseelie courts, sidhe, slaugh. I want goblins and redcaps to be the foot soldiers of the unseelie court, striking out at the enemies of the Queen of Air and Darkness, or just whoever pisses her off. Eberron's changeling's probably fall into this category somewhere.
Things I've done with the Fey, etc.
1) Extraterrestrial Greys (Asgardians from SG-1, Close Encounters, Fire in the Sky, X-Files, etc.) doing experiment on primitive beings on the planet crashlanded during a low level mission. However, they rescued enough of their engineering tech to create a new living space under the mound created by their crash so many centuries ago. The rescued tech created a hyperdimensional space and allows them to slow the passage of time, giving them seemingly limitless time to work on the problems facing them...like how some of their experiments have become fully sentient.
(This lets you have a different explanation for how visitors to Underhill experience disorientation as to the sheer size of the place, a percieved difference in the passage of time, changelings, and "magically altered" associates of the people under the mountain.)
Few adventure outside of their settlement, but those that do are noticed for their unearthly beautiful slanted eyes ("like looking into a starry night"), slender limbs, and command of unusual mageries (actual tech, magic or psionics, your choice).
2) A more conventional approach like your Changelings would be to combine features of those with some of the Elves' traits...and then add in Geomancer-style Drift.
*I also want to include some Native American supernatural flavor. peoples like the manitou (perhaps the rest of the fairies and elves that' don't fit into the seelie/unseelie courts. Wild elves definitely belong here. Lead by their own race of gods the Gitche Manitou or Wakan Tanka, who definitely have names like "the white buffalo woman", "old man coyote" and "grandmother toad"
A 2Ed PC of mine was an albino Minotaur Ftr/MU from a Plains Indian culture (inspired by the art from the first Hurloon Minotaur M:tG card), using chariots instead of mere horseback. His unusual size and coloration marked him for having a great destiny as a leader among his people...
In a 3.X environs, consider having a predominance of the divine casters in the culture to be something like a CompDiv Spirit Shaman or OA Shaman. Also consider the Kalamar Shaman or the AU/AE Greenbond.
Use totemic barbarians/rangers/scouts, etc. as depicted in UA, or AU/AE as the predominant warriors, and tweek their skillsets & class abilities to reflect their regions- archery & Riding related skill bonuses & bonus feats among the plains culture, for instance, or skills & feats more appropriate to a waterborne lifestyle for those along big rivers, lakes or oceans.
I'd also check out Deadlands (esp. the D20 version) for some inspiration.
