Older versions of Waterdeep made the city slightly grittier. Geographically it's loosely modeled after Lankhmar from the Fafhrd and Grey Mouser swords & sorcery stories by Fritz Leiber. Greenwood made it much more friendly, with a colorful high-fantasy setting. However, you could choose to make the place grittier by modeling it after Lankhmar.
It's a city of Thieves infested with crime. There's a powerful Thieves' Guild that tries to control all criminal activity. There are extremes of wealth and poverty. The nobles live in a district gated off from the rest of the city with lots of private security. In order to survive, most people are forced to engage in morally grey behavior. There's a street of the gods, where the gods regularly move up and down the street, based on how many worshipers they have. They constantly jockey for position to get into the fancier temples. If they run out of worshipers, they'll eventually get kicked so far down the street, they get kicked out of the city through the "gate of the gods".
The characters engage in intrigue, heists, stumble onto ancient eldritch magic (and horrors), etc.
You could probably crib some stuff from that setting, and plug it into Waterdeep to make the place more of a powder keg, where people are less secure, and are more likely to attempt a revolt.