A Challenge! - Notes on a New Setting

My oldest son and I had a real-life adventure yesterday. we had to drive 100 miles or so to pick something up for friends and on the way back the van stalled. One good samaritan Southern gentleman, a roadside assistance program complimentary tow, and a cargo transfer later and all was well .
The fact my oldest is also a certified mechanic means the total repair cost was zero, too.

During this 10 hours of driving, hauling, waiting, waiting, lifting, waiting, and waiting my son issued a mild challenge.
My primary campaign, Seaward, uses AD&D 1e in a setting that is a mish-mash of European fantasy with steam-powered horses, fire lances, and merchants from the moon.
My secondary campaign, Blackstone, is a very standard European fantasy setting with some Oriental, Middle Eastern, and Jungle settings.
He challenged me to make a setting that is totally "non-standard".
MY PLAYERS SHOULD STOP HERE
I am going to have the players make 3rd level characters using AD&D 2e Skills & Powers. I am working on unique versions of the various classes that they can play. No generalist mages, only specialists (force, geomancy,  - the only generalists are based on Al-Qadim sha'ir. Clerics do not exist, instead a version of the psionicist from the Skills & Powers books that have limited power access to the psychometablism and similar power sets for healing, remote sensing, etc. to fulfill that role.

The players will begin in Hubtown, an underground city of about 2,000 people where 9 tunnels meet. A center of frontier commerce for the R'tern Empire, Hubtown gives access to the Bandit Caves, the frontier tunnels, farming pits, and even The Valley Above.
The Valley lies between two impassable mountain ranges that stretch so high they vanish into the sky. While full of things like fruit, game animals, and quick access to entrances of tunnels that can take days to reach otherwise, it swarms with everything from poisonous snakes to the z'rrt, the oord, and the terrifying izkat.

The party will be looking for loot, fighting bandits, and hoping to fins one of the ruins left by the Old Ones, full of incredible magic and artifacts.

My players should really stop here

EDITED FOR CLARITY The adventure will appear to be inside an O'Neill Sphere - a massive hollowed-out asteroid spun on an axis with a spherical hollow within.  The Valley is the equatorial region and the mountains are where the sphere tilts away from rotation. The sun is a device that shines collected sunlight inside.
Very Vancian Dying Earth, the inhabitants forgot the nature of their world 10,000+ years ago, leaving the mutants, genetically-engineered animals, and repairing equipment to myth.
  If I run is carefully the reveal will be great because - it is actually a node on the Plane of Earth settled by teleportation! That's right - not an asteroid, not spinning - it is an intrusion of Elemental Air and Fire into the Elemental Plane of Earth that looks enough like an O'Neill sphere to make the players assume I am going high-tech when I am not.