The premise is getting refined in my stray moments. The elements and history that might have lead to the mandate for such a situation are still loosely defined. At this point, I envision a surface world consumed by fire, probably demonic in nature. Using my concepts for portals that connect worlds (which would require massive scanning and text conversion of the old CoT's to do any justice to), I envision a portal being manipulated, from one side or the other, and creating an egress that once opened, spilled hellfire across most of the surface of the world.
Original ideas and some dated sketches of this concept show a few mountainous zones that due to altitude, geology, and/or geography, escaped the wrath. One such elevated location would have a portal on site, and either due to security or issues of alignment and tuning, was not able to be utilized as a hellfire portal. Further, minor orbiting sphere/worlds (again, detailing this is much scan and text conversion away) connected to this world would have been subject to the hellfire consumation as well, barring a few that were on the same 'secured' channel access that the surviving planet-side gate was on.
So to summarize, this is a world mostly consumed in fires, with a few patches of miserable surface world left standing above the hellfire. Orbiting it are a number of small 'spheres' most of which are consumed in fire as well. A few orbiting sphere-cities escaped, the last vestige of normal existance left to this world.
Now, for whatever reason, left to later musings, the technology existed on this world that also opened subterranean spheres up. I envision it working something like this: Deep shafts were bored into the earth, and suspended down into the space was a sort of energy converter... it created a spherical field of energy and glazed the rock solid, and then, as the field's size adjusted, vaporized everything inside the shell, converting it to energy and compressing it into a hyper dense mass, which would later become part of the fuel core for the 'sun shard' that would make the space habitable.
Once the space was properly glazed and vacated, engineering teams would decend into the space and begin constructing a secondary shell of arches and anchors that would act as a support for the cavernous roof and a mounting for the structure that would suspend the 'sun shard' in the center of the void.
In an ideally built city, the upper tiers, built between the heavy arches and the glazed shell would probably be rather elite housing, with lower built homes haveing similarly lowered status. The notion of suspended shanties and breaks in the shell leading to dark alleys would apply to broken, failing, or improperly built cities only. Also, the better cities would have ample gardens, hanging and trellising down and around the space, making for a lush, humid, and fragrant space. Older or broken sites would have much less greenery, and feel dry and thin aired.
Further, I envision a series of tunnels, perhaps with some sort of 'subway' system, connecting many of these deep cities... as well as many abandoned tunnels and caverns.
In some cities, the Sun Shards may be suspended entirely from above, via huge chains. In others, mainly the more run down cities, supports from below are used to secure the shard in place (as previously envisioned).
Ideas about this are swirling madly at the moment, but this is where I am at for the most part. More to follow I am sure.