The Forgotten Realms is the second worse setting in Dungeons and Dragons, though I do agree that Ravenloft rocks.
I am not going to argue over sewer architecture on what should be Realmsian and what is medievel and blah blah blah. Hell, even in the Realms sewer architecture is different from region to region. I don't think the cities of Thay would use the same style as the city of Waterdeep, you know.
So instead of having a basic generic sewer tileset that can work in any medival campaign, FR or not, lets have FR specific ones for each region/nation in Faerun. Yeah, thats a good idea.