The main idea is that you take the already existing races and you find a way that an already existing character could get his race tweaked Through a series of quests. This would not offer too many mechanics changes, just a few extra racial abilities and an edit to model.
A character gets enough fel energy into it's body to cause a demonic mutation. The fel orc is the easiest example to get into. An orcish character goes through a number of quests that culminate in the character drinking some pit-lords blood, thus becoming a Fel Orc. All that changes is skin tone and a few added racial abilities appropriate to the change. Examples in other races could include Night Elves slowly changing to satyrs (perhaps not all the way more gradual), or Dranei growing more towards demonic Eredar (red skin) or Broken.
Orcs could revert more to their brown skin-tones or possibly more. Night Elves could start growing antlers.
A character gets bitten by a worgen and starts a quest to master the curse of the worgen, culminating in them being perfectly normal, just being able to change into a worgen at will just as a druid changes into a bear or a cat.
A character is infected with the Lich King's plague, and masters the unholy effects on him/herself. Examples would be when the character dies for a short period of time the corpse rises as an undead and fights (similar to a priests ability) or has the ability to infect enemies, causing a similar effect upon their death. Visual modifications would be warts, decaying skin, or a generally deathly appearance.
Largely geared towards dwarves and gnomes, although possibly available to others, this would give a character skin of stone, stacking with the dwarves stoneskin ability and allowing them to change to iron skin. Obviously a boost to constitution, armor, and resistances.
The player could become infused with the energies of the Twisting Nether and become ethereal, like the black dragons did to become nether dragons, and gain abilities like being harder to damage with physical attacks while out of phase.