I picture Hyborian cities as being crowded, unsanitary places, like many cities of the Middle Ages: narrow, winding, cobbled streets between brick buildings, little or no sewage system, open-air food markets and bazaars. Cities would be surrounded by open farmland for crops, orchards and animals.
Stygia I imagine to be like Teotihuacan, Macchu Picchu, and a number of other Aztec, Toltec, and Mayan cities combined. The dominating them would be the huge plaza with several step-pyramids and temples to various gods, arranged according to star patterns and used at specific times of the year for worship or for training the religious elite.
In the more exotic locations--remote, forbidden islands, or places deep in the jungle--there are the remains of ancient civilizations: cyclopean statuary and huge blocks of carved stone, broken and tumbled from cataclysm and age, half-buried in the ground and covered in moss and vegetation--but still retaining an air of mystery and evil.
Like you, I imagine that the greater part of the cites were crowded filthy areas to be living in. However, in several stories mention is made of temple and palace areas. I have always imagined these areas as being more open. Wider streets and better lit. These areas would be controled by those with both power and money so they would want there areas to be places that could show the degree of power and wealth they had achived.
To that end, these are the places that have the great domes and towers that is most often mentioned in the stories. Buildings with walls of granite and marble, statues used to show past past greatness, sewers, and roads that were laid out with paving stones.
Once you were out of these areas you found yourself in places were the buildings were only 2 or 3 stories in height. Roads and alleys that were little more then trampled earth and mud, piles of refuge building up till the next heavy rain could wash them into the lower parts of the city.
I do like the concept of Stygian temples being used on a rotating bases. while set was the primary deity in the land, we do have some stories showing that He was not the only god worshipped in the land. If the stygians had a cycle of gods, it makes sense that some could be in asendence at different times of the year while others were in decline.
I've never seen Luxor or Khemi as South Amercian style cities. Instead I see them more as a combination of old Egyptian and Middle Eastern styles.
The forgotten areas I agree with you completely. Ancient stone work, cracked and rounded with the weight of years and nature. Collapsed monuments covered with vines and moss, becoming bizare formations. Weathered carvings worn smoth by wind and rain till they are little more then indentations apon the crumbling stone.