The main areas that still need to be detailed are the relatively smallish kingdom of Tarush Noptii, a kingdom ruled by vampires, and the fractious successor states to the faded glory of Baal Hamazi. Baal Hamazi isn't a fading empire in the sense that Terrasa is, it's an empire that's completely gone. Nothing is really left in the traditional sense; the remnant states are to Baal Hamazi as the city-states of Renaissance Italy are to the Roman Empire. Many of these successor states still claim to be the "true heirs" of Baal Hamazi, yet none of them are truly capable of understanding the grandeur that is their social ancestor, and their interpretations of what it means to be Baal Hamazi reborn differ dramatically.
In part because of this, the successor states are a highly Balkanized and politically charged area; nobody has been able to amass significant power in several generations, because the roots of conflict between the various states are too deep and too difficult to overcome.
The idea for Baal Hamazi comes from the 4th edition implied setting, actually, and the name I choose echoes that influence. Bael Turath is the 4th edition tiefling empire of history, and Baal Hamazi is the same. For whatever reason I've always liked the concept of tieflings, even before they existed under that name (Shakespeare wrote of Caliban, who is essentially a tiefling, and Merlin was said to be one as well.) I didn't want to use the name tiefling (which is too bad, because it's a good name... but I don't want such clear and obvious ties to Wizards of the Coast's intellectual property) so I've been casting about for a substitute. Cambion is actually an old Medieval word that refers to the child of a human and a succubus or incubus. Hellkin is another one I've tossed around (and gradually become more dissatisfied with) and darkling is a literal translation of what the word tiefling would be in German. The story is that around the TSR design studios, someone asked Wolfgang Baur to come up with the name, and he picked a German word that basically means darkling. But the idea of a culture of humans who has been tainted by the blood of fiends, and who proudly bear that heritage, intrigues me. It's a very Sword & Sorcery compliant idea.
Although the idea of a former tiefling empire came from 4e, the specific visualization of them does not. In my setting, there are two kinds of tieflings (well... whatever I end up naming them finally); those who are native hamazin, from Baal Hamazi, and those who are changelings, born unexpectedly amongst the other cultures and population groups of the settings. The hamazin have started breeding true, after many generations, and have settled on a common physical appearance. Obsidian black skin, yellow eyes, and dark hair are the hallmarks of the hamazin, as well as a cluster of small horns on their heads. I initially was going for a Graz'zt-like look, but it occurs to me that they also resemble Star Wars zabraks. In fact, Darth Maul, without the red stripes (and without being obligatorily bald) would be a perfect example of a hamazin. Maris Brood, from the game Star Wars Force Unleashed would also be a good one, except that of course she needs to have jet black skin. I've attached a picture, because she's a good example of almost the look I'm going for.
Now the changeling type, on the other hand, do not have a common physical appearance, and can be much more varied in their appearance. An important character I've come up with, Francesca de Sperança Domènechoz (a Terrasan, born in the rural former colony of Calça, with a twin brother who is a regular human) is a good example of one of these, and as a young woman, she has ash-gray skin, white hair, and solid milk-white eyes that look like they're caked with cataracts, but which actually see very well, even in the dark. Others may have even more disturbing physical features---forked tongues, prehensile tails, needle-like teeth, scaly skin, or worse.
Anyway, as soon as I settle on a name for my analogs of tieflings, I'll post my houseruled version of them. It is important to me that they be an LA+0 race, rather than an LA+1 race, so I do not use the rules for tieflings from D&D itself; just the concept.