So, to make your own N.C.:
1) Take your three favorite human-ish monsters out of the Monster Manual and they are minority citizens in the city. Detail how they get along, how being in the city has culturally changed them and what niches they fill in the city. How do the powers that rule the city keep them down?
2) Take three really bizarre fucking monsters and figure out how they exist in the nooks and crannies of the city and how the powers that rule the city keep these beasts from doing unacceptable amounts of damage?
So, I’ll do one.
1) 3 monsters: Githyanki, Minotaur and …something not quite so martial…maybe…sprites.
Githyanki: These Githyanki are refugees, AWOL soldiers from countless planar wars and whole military companies who were cut off from communication with the Liche Queen’s grace. Some still worship her and pray for her to make contact (or even take this plane ever in a barrage of silver swords and red dragon fire) and others in an attempt to rebel against their upbringing and actually have adopted their own approximation of Githzarai meditation techniques.
The Githyanki are forbidden from carrying any weapons larger than a knife, due to a paranoia that their Liche-Queen will one day contact them and they will rise up and attempt to take the city by force. Due to this, many carry silver knives on their hips, in memory of the silver swords of their past.
Minotaur: The minotaur have built many of the winding alleys of the city, leading it to be almost unmappable and very difficult to navigate. This pleases the minotaur aesthetic, as they not only lived in labyrinths in their home country but their religion worships the winding roads that life leads you on. The city’s powers are slowly starting to elevate minotaurs into civic posts, especially as more and more Githyanki refugees are moving into Minotaur neighborhoods. The minotaur are being cast in the role as the city’s valiant defenders against the vile foreigners.
Sprites: As the city has overcome their groves, the sprites have adapted. They treat building roofs and water containers as their groves now and refuse to live indoors. Their inability to live indoors causes the city’s elders to treat them as illegal immigrants from the land of Faery. Many rich folk even use hawks to hunt sprites as they are seen as sub-human and not fully sentient.
2) Illithid: The city’s elders have a deal with a cabal of Illithid in the sewers, discreetly sending down those in poverty, orphans or homeless who they assume none would miss to feed their brain-hunger. In return, the cabal keep to the sewer deeps and send word of rebellious thoughts from above.
Otyugh: These beasts are use to consume garbage, tamed by those who collect trash and utilized by the underworld to devour bodies.
Dragon: There is a dragon sleeping in the Boiling Bay, and has slept there since the city’s founding. Every season, treasures are put into the bay and allowed to sink to the bottom so that the dragon’s hoard will be great, giving it sweet dreams and deep sleep.
If someone could make their own N.C. using the above method, that’d be keen.