If it was implemented, world shape could be handled easily enough by an advanced world gen parameter, like polar region placement. Spherical worlds would raise some questions about how the map handles wrapping, though. Would certain areas actually be smaller than they appear on map, to represent that a round object is being represented on a square medium? Or would it all be 1:1?
In any event, where would hell come into play in this?
- In spherical worlds, would it be its own sphere, wrapped in an adamantine bubble with a crunchy planetary coating?
- Would a flat world have its underworld hanging freely under the main world, and would either one be walled dependent or independently if the other?
- Would it be its own world connected to the main one, with magical portals deep underground that just happen to be surrounded by veins of adamantine?
As for other celestial bodies and greater cosmic structure, unless there is a means travel to other celestial bodies (Like that time/acid-trip/fever-dream where the Khajiit allegedly managed to reach the moon), or a means for celestial bodies to come to us (Earth-Theia Giant-Impact Event reenactment, a.k.a That time we got Majora's Mask'd) it all might be better left as an abstraction, data that comes into play when astronomy and other relevant things need it. Generating one medium-sized world is time-consuming enough, I would rather not to wait the life-span of an actual universe for the creation of an artificial one.