Here's a whirlwind summary of my campaign setting for those who have expressed interest. I'm posting it because of interest shown in another thread (the feat categories one, IIRC).
This is an overview, but to keep it on-topic for this board I'll try to keep the emphasis on how I've integrated the True20 mechanics and where (and why) I've deviated.
The heart of the setting are the Celestial Spheres. Each race has their own, unique mythos that explains how and why the spheres where created, how the various races and planets are tied to them and why they are the "most important" race. The PC races are referred to as the mortal races.
There are three Spheres of Creation - Nature, Self, and Heaven. These provide mortals with bodies, minds, and souls, respectively. Undead lack links to one or more of these spheres - so vampires are soulless undead, while ghosts are bodiless undead.
There are three Spheres of Perfection - Conflict, Knowledge, and Music. These provide mortals with the mastery over their environment and are considered the way to refine and perfect the body, mind, and soul, respectively. Most creatures apart from mortals lack links to one or more of these spheres.
There is also the Sphere of Intrigue. This mysterious sphere grants mortals free will (which unfortunately includes the ability to lie and cheat). Lacking such a link, fey do not have free will and are therefore unable to lie (though they can be devious, they cannot speak untruths).
The link between mortals and spheres varies in strength. A normal link simply is - the mortal gains the basic benefit of the sphere merely by existing. A deeper link is usually called a bond. Mortals are bound to specific spheres by both birth (each race carries a bond to a specific sphere) and by choice (each role forms a bond to a specific sphere). Mortals bound to the same sphere through both birth and choice are said to resonate with that sphere, and gain additional benefits.
Individuals can cross from the mortal world into the spirit world. Fey are native to the spirit world but can cross freely into the mortal world. The curtain between the two worlds is called the Veil. Some important locations, such as the City of Brass and the Faerie Court are located in the spirit world, though mortals don't typically know this as they don't know when they've wandered across the veil.
So that's the basic cosmology. As a very quick guide here's a summary of the seven Spheres and things tied to them.
Sphere of Nature - the moon, silver, the body, the natural world, animals, plants, fey, elves (PC race), druid + wildlander (roles)
Sphere of Self - the earth, stone, the mind, reptiles, unarmed combat, naga (PC race), psion + inkyo (roles)
Sphere of Heaven - the stars, mithril, the soul, luminous (PC race), crusader + runemaster (roles), truenames
Sphere of Knowledge - the sun, crystal, brass, perfects the mind, giants (PC race), scholar + mage (roles)
Sphere of Conflict - iron star (rust-colored planet), iron, steel, dwarves (PC race), orcs (extinct race), guardsman + marksman (roles), perfects the body
Sphere of Music - music of the spheres (the stellar void is believed to be filled with music), jade, the sea, ooloi (PC race), bard (role), perfects the soul
Sphere of Intrigue - hidden planet (hidden between the spirit and mortal worlds;wild card for astrologers), free will, deceit, betrayal, humans (PC race), ninja + courtier (roles)
Witches (role) stand outside the spheres, linked to all but bound to none. They make mortals of the same race uneasy because they are different at an unconscious level. It is said that they "stand between"; between mortal and spirit, between life and death, etc.
Arcana and feats are also bound to spheres. Practically this is more an organizing principle than any real mechanical benefit, but I expect this to change once some of the other mechanical issues have been ironed out. Lots of new feats adapted from d20 sources; a handful of new arcana.
There are 14 roles in all; each role gets special, unique benefits at levels 1, 5, 10, 15, and 20. These are also the levels at which you can change roles. Mortals cannot be bound to more than three spheres, so there is a practical limit to the amount of multi-classing you can do. The True20 roles were used as a reference when balancing the roles.
There are racial levels (aka racial paragon levels in Unearthed Arcana) - I'm still balancing these but currently you give up a feat when taking one; you gain some other benefit instead (such as an ability boost). Elves who take three racial levels become fey and lose their free will - they can be summoned or possessed by the Faerie Queen at any time, can no longer lie or cheat, and must undertake any quest or order given by the queen.
Each culture has its own weapon and armour list; most nations are at bronze-age levels, though elves and ooloi have stone age technology and dwarves reached medium iron age levels before their empire collapsed (thanks to disease).
Many creatures of legend are just that; mermaids and djinni do not exist, but are really distorted accounts of encounters with ooloi and giants, respectively. Others are real but extinct (like orcs), some are real but live in the spirit world (unicorns) and a few are probably real but not seen in living memory(dragons? the phoenix?)
Other rules that have been adapted or are in use - debate rules from Dynasties and Demagogues, completely new craft system, material and degradation rules from Stone to Steel, new language rules (most mortals cannot read, DCs and guidelines for language checks), real-world poisons and effects from Assassin's Handbook. Some of these are still being playtested.
Happy to answer questions but will try to keep this at a design level; I don't want this to turn into an advertisment, you know?