The elementals... magically created creatures specially adapted to survive in environments that normal creatures can't. They're still somewhat vulnerable to Devourers, but the other Demon Legions can't really do much to them. The most common elementals are Salamanders and Undines.
The Fortress Realms: much like the Upper Realms, this is a small cluster of tightly connected realms. Unlike them, however, the Fortress Realms have bottled themselves up behind endless barricades and traps. It's hard to say what the quality of life is like inside the Fortress Realms, or even to confirm that they really deal with each other. They're terrified of giving away information to outsiders, so they rarely even communicate with outsiders. Serious attempts to get in cause them to launch devestating "counter"-attacks... so there are people inside, but they aren't talking. At least one Fortress Realm has been found that was utterly abandoned, though, apparently having been taken over by Devourers and wrung dry.
Mercenary Legions: there should probably be at least one big mercenary group... a legion of powerful warriors and mages who make their living by fighting for one side and then the other. They're generally paid in slaves, food, magical enchantments, etc.. Of course, they can't get work from the Devourers or Ravagers, but there's plenty of other employment... and work for PCs who are willing to join up. It may, in fact, just be a placement service with a tiny realm that's still on speaking turns with all sides and provides mercs to them all. Heck, they could be headquartered in Hybris, although that might make the city too corrupt... I'd kind of prefer a Hybris-like fortress that deals in slaves and such, dunno. That might be something for the second version.