Songs of Erui is the first long-term homebrew campaign that I've tried to run in a long time. It was largely inspired by the works of Robert E. Holdstock (Lavondyss, Mythago Wood, and The Hollowing), Hellboy, Princess Mononoke, and reading the chapter on The Feywild in Manual of the Planes. I decided that I wanted to do a campaign with lots of heavy celtic influences, and with Player's Handbook 2 rolling out the primal power source it seemed like a sign.
Using Wikipedia I brushed up on celtic mythology to mine for ideas, changing what I needed to best make it all fit within the "points of light" concept and default D&D assumptions (such as monster origins, history, and cosmology). I wanted to emphasize the existance of primal races and classes, but not discount anything. It was an interesting exercise, and helped a lot in providing me with a rough framework for levels 1-20 (at this point, Epic tier is still being a bitch).
The basic idea is that Erui is the original birthplace of many primal spirits (might be all, I havent fully mapped out my homebrew world). Its a wild cradle of life that is isolated from the rest of the world, located far north of Nerath. It is separated by a massive mountain range that is said to be so tall that if you climbed to the top of its peaks that you would reach the Astral Sea. A ruined bridge called the Dragon Road cuts directly through the mountains, but those that walk this ancient path are never heard from again, and no one knows why.
The land is sentient to a degree. Excessive damage to Erui can cause it to conjure up massive animal spirits that defend it (like the animal gods in Princess Mononoke). The forests can cause you to get lost or transport you great distances through time and space (Mythago Wood). Wierd stuff like that thats not really covered at all in the rules. Of course, its also defended by more tangible threats like malicious fey.
Natives of Erui include primal and/or savage races such as gnomes, elves, eladrin, goliaths, gnolls, and (of all things) devas. Devas have a special place in Erui that coincidentally ended up being almost identical to what was described in Ecology of the Deva.
However, since I wanted to allow every race I had it so that several hundred years ago many other races migrated north for whatever reason: pilgrims seeking a "holy land", explorers, or refugees trying to escape the war that was consuming Nerath.
Whatever was necessary to get them up there, and in the end dragonborn and dwarves became some pretty major players.