So, firstly I'd like to say that your chosen backdrop for a campaign is just -magnificent- and I love it to pieces and I want to play in it. Please post tons of lore and information about it as you go. I -beg- of you!18 years later and we're about to see the 5e reinvention of the setting. I'm thinking about building another dark domain based on a fog-filled analog of Victorian London. A sinister secret society grows like a cancer in the body politic, enacting occult rituals and sacrifices in the dark corners of the city. Lightless alleys, haunted sanitoriums, crowded cemeteries, effluvia filled sewers, and gothic cathedrals crawl with horrors both real and imagined, manifested by the forbidden rites enacted by this mystery cult. The PCs wage a war of shadows on an unknown enemy that stains the fabric of society. Who holds power? What is their aim?
It seems like the most obvious, trope-y gothic horror sort of setting but as far as I can remember there isn't really a version of archetypal urban horror in Ravenloft. I'm reading through the Red Box right now to jog my memory and see if there's not something super obvious that I'm missing.
My biggest world-building issue pops up when I look at existing domains, realizing both their size and populations. Most domains are small and their population are absolutely tiny compared to 19th century London. How would a city of millions eat? If it's an industrial center like London was, how does that economy even function in the context of the Core in Ravenloft? It's really going to bother me to be super hand-wavey about the world building aspect here but it seems hard to drop a big urban domain into the setting and have it make sense.
Secondly... Trade.
Plop this badboy at the intersection of Falkovnia, Dementlieu, Mordent, and Richemulot.
Kill the road from Mordentshire to Chateaufaux and instead make a road from Mordentshire to your place. Same with the link between Chateaufaux and Falkovnia, make them come through your place. Silvervas and Pont-a-museau? No road, there. It goes through Victorian London Horrorland, now.
Your town is now a Crossroads Realm that Falkovnia desperately needs to get it's bread-basket goods to three other important domains in exchange for weapons, armor, and soldiers to battle against the endless zombie horde that wipes them out every once in a while, but -somehow- never interrupts the monthly shipments of food through your town.
Which, of course, places Tariffs on everything that the different realms you're connected to mostly pay through food.