Some initial thoughts:
Fernia, the Sea of Fire
The efreeti City of Brass is built around a group of unusually stable portals leading to various other planes. Through their strict control of these portals, the efreeti have become the greatest political power of Fernia as well as masters of planar trade. However, the quori have their eyes on the city, and no one's sure how far their plans have advanced.
Some efreet breed (or perhaps farm) fire elementals by carefully tending the maelstroms in which they are spawned, selling exceptionally strong or obedient elementals for use as mounts or to enterprising elemental binders. A novelty market exists in elementals of unusual shapes and colours.
The efreet look down on other races (unless they're customers), with a legal system which restricts the rights of non-efreeti citizens - a number of rebellions have taken place, but none have met with success. *insert notable firre eladrin*
The Scarlet Feathers is an organisation resembling something between an organised crime group and a religious cult. Its leader, Grefis the Grand (NE fiendish phoenixMM2), promises his predominantly phoelarchMM3 clerics freedom from efreeti oppression and rebirth as true phoenixes. However, he has neither the ability nor the interest in doing so. Due to his vanity Grefis avoids physical confrontation at all costs, for fear that someone will ruffle his feathers.
A great spherical mechanism, seemingly of Giant construction, floats within Fernia like a sun. At seemingly random intervals (ranging from days to decades), sections will rotate and reconfigure, creating a set of entrances and passageways which is never the same twice.
It is said that deep within lies the true heart of Fernia, a sea of golden flames which can be harnessed for ultimate destruction or ultimate purification (making this an appropriate location for artifact creation/destruction and character rebuild quests). Explorers from numerous factions across the planes can be found within the sphere, despite the risks, some having been trapped for centuries when the exits suddenly moved. Besides that, the sphere is inhabited both by ancient guardians and colonising beasts.
A handful of red and gold dragons have settled nearby, believing the rotations of the sphere may be linked to the Prophecy, and sometimes sponsor exploration parties.
Shavarath, the Battleground
Why is everyone fighting, besides hating each others' guts? Does something bad happen if demons occupy a particular region?