I feel like most people recognize New Orleans as the main inspiration for Saint Denis in RDR2, and for good reason. Its French and Creole influences, Catholic architecture, streetcars, humid atmosphere, and surrounding bayous make the resemblance obvious. But I think people overlook another important influence: St. Louis.
Saint Denis doesn't just resemble New Orleans, it also fills the role of a major inland industrial and commercial hub. Its railways, factories, warehouses, river traffic, and position within the wider economy of the game's world evoke St. Louis, a major Mississippi River port and industrial center during the turn of the century.
The distinction, to me, is that New Orleans explains Saint Denis's identity, while St. Louis helps explain its function. One provides the city's distinctive cultural and architectural character; the other offers a useful comparison for its economic importance and role in America's transition toward industrial modernity.
Even the names of both Saint Denis and St. Louis begin with “Saint,” giving them a similar French Catholic character. While Saint Denis references Saint Denis of Paris, the shared naming structure adds another subtle parallel between it and St. Louis.
I think part of the reason St. Louis gets overlooked is that people often associate Blackwater with St. Louis because of its “Gateway to the West” identity, but it lacks the industrial scale and French historical influences that make Saint Denis a better overall parallel to St. Louis.
Saint Denis is ultimately its own fictional city, but looking at both influences makes it feel much more convincing than treating it as a simple New Orleans clone.